Wavelength monitor and semiconductor laser device Number:6,801,553 from the United States Patent and Trademark Office (PTO) owispatent A wavelength monitor comprises a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.<H semiconductor, Wavelength, Wavelength, moni, 6801553.html, Patent, pto, 6801553

Title: Wavelength monitor and semiconductor laser device

Abstract: A wavelength monitor comprises a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.

Patent Number: 6,801,553 Issued on 10/05/2004 to Imaki, et al.

Inventors: Imaki; Masao (Tokyo, JP), Hirano; Yoshihito (Tokyo, JP), Sato; Makoto (Tokyo, JP), Masuda; Kenji (Tokyo, JP), Adachi; Akihiro (Tokyo, JP), Nishimura; Yasunori (Tokyo, JP), Takagi; Shinichi (Tokyo, JP)
Assignee: Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)

Appl. No.: 09/988,662

Filed: November 20, 2001

Foreign Application Priority Data


Dec 06, 2000 [JP]			2000-371471
Apr 27, 2001 [JP]			2001-132746

Current U.S. Class: 372/32 ; 372/29.02

Field of Search: 372/29.02,32

References Cited [Referenced By]

U.S. Patent Documents


3951509	April 1976	Noguchi et al.
4998256	March 1991	Ohshima et al.
5036185	July 1991	Ando
5095476	March 1992	Greve et al.
5224084	June 1993	Takahashi
5559767	September 1996	Matsui
5825792	October 1998	Villeneuve et al.
6101211	August 2000	Wakabayashi et al.
6181717	January 2001	Kner et al.
6272157	August 2001	Broutin et al.
6301216	October 2001	Takahashi
6542664	April 2003	Srinivasan et al.
6587214	July 2003	Munks
2002/0061039	May 2002	Le Gall et al.

Foreign Patent Documents


58-12831	Jan., 1983	JP
63-193004	Aug., 1988	JP
5-149793	Jun., 1993	JP
10-79551	Mar., 1998	JP

Primary Examiner: Wong; Don
Assistant Examiner: Al-Nazer; Leith
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis, L.L.P.

Claims

What is claimed is:

1. A wavelength monitor, comprising: a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough; first and second photodetectors configured to receive the laser beam passed through the cylindrical lens; and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector, wherein the wavelength filter is disposed outside an optical path between the semiconductor laser and the second photodetector.

2. A wavelength monitor according to claim 1, wherein the wavelength filter is disposed in the optical path between the cylindrical lens and the first photodetector.

3. A semiconductor laser device, comprising: a semiconductor laser configured to emit a laser beam; a cylindrical lens configured to allow the laser beam emitted from the semiconductor laser to pass therethrough; first and second photodetectors configured to receive the laser beam passed through the cylindrical lens; and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector, wherein the wavelength filter is disposed outside an optical path between the semiconductor laser and the second photodetector.

4. A semiconductor laser device according to claim 3, wherein the wavelength filter is a birefringence filter.

5. A semiconductor laser device according to claim 3, wherein the wavelength filter is a birefringence filter comprising a YVO.sub.4 crystal, a LiNbO.sub.3 crystal and a polarizer.

6. A semiconductor laser device according to claim 3, wherein each of the first and second photodetectors has an elongated beam receiving face and wherein an elongation direction of each of the first and second photodetectors is oriented perpendicular to a center axis of the cylindrical lens.

7. A semiconductor laser device according to claim 3, wherein each of the first and second photodetectors has a rectangular beam receiving face, each rectangular beam receiving face having four sides, two of the sides of each rectangular beam receiving face being perpendicular to a center axis of the cylindrical lens.

8. A semiconductor laser device according to claim 3, wherein the wavelength filter is disposed in the optical path between the cylindrical lens and the first photodetector.

9. A semiconductor laser device according to claim 3, further comprising a beam shielding plate which has an aperture and which is arranged between the semiconductor laser and the first and second photodetectors.

10. A semiconductor laser device according to claim 3, further comprising a positioning member having two plane surfaces that contact the cylindrical lens.

11. A semiconductor laser device according to claim 3, further comprising a positioning member fixed to the cylindrical lens with a gold-tin alloy or with a glass material having a low melting point.

12. A semiconductor laser device according to claim 3, wherein at least one of the first and second photodetectors has a plurality of photodiodes.

13. A semiconductor laser device according to claim 3, further comprising: a temperature-keeping device to which the semiconductor laser is directly or indirectly attached; and a control circuit configured to control the temperature-keeping device according to a ratio of an intensity of the laser beam received by the second photodetector to an intensity of the laser beam received by the first photodetector.

14. A semiconductor laser device according to claim 3, further comprising: a temperature-detecting unit configured to measure a temperature of the semiconductor laser; a temperature-keeping device to which the semiconductor laser is directly or indirectly attached; and a control circuit configured to control the temperature-keeping device according to the temperature of the semiconductor laser measured by the temperature-detecting unit and according to a ratio of an intensity of the laser beam received by the second photodetector to an intensity of the laser beam received by the first photodetector.

15. A semiconductor laser device according to claim 3, further comprising: a temperature-detecting unit configured to measure a temperature of the semiconductor laser; a temperature-keeping device to which the semiconductor laser is directly or indirectly attached; and a control circuit configured to control the temperature-keeping device according to the temperature of the semiconductor laser measured by the temperature-detecting unit and an intensity of the laser beam received by the second photodetector.

16. A semiconductor laser device according to claim 3, further comprising a control circuit configured to control the semiconductor laser according to an intensity of the laser beam received by the second photodetector.

17. A semiconductor laser device according to claim 3, wherein the first photodetector and the second photodetector are disposed adjacent to each other in a direction parallel to a center axis of the cylindrical lens.

18. A semiconductor laser device according to claim 17, wherein a beam diameter of the laser beam passed through the cylindrical lens in a first direction parallel to the center axis of the cylindrical lens in a plane that includes a beam receiving face of the first photodetector and a beam receiving face of the second photodetector is longer than a summed length of both the beam receiving face of the first photodetector and the beam receiving face of the second photodetector in the first direction, and wherein a beam diameter of the laser beam passed through the cylindrical lens in a second direction perpendicular to the first direction in the plane that includes the beam receiving faces of the first and second photodetectors is shorter than a length of any of the beam receiving faces of the first and second photodetectors in the second direction.

19. A semiconductor laser device according to claim 3, wherein the beam receiving faces of the first and second photodetectors are inclined relative to a plane perpendicular to an optical axis of the semiconductor laser device.

20. A semiconductor laser device according to claim 19, wherein a beam diameter of the laser beam passed through the cylindrical lens in a first direction parallel to the center axis of the cylindrical lens in a plane that includes a beam receiving face of the first photodetector and a beam receiving face of the second photodetector is longer than a summed length of both the beam receiving face of the first photodetector and the beam receiving face of the second photodetector in the first direction, and wherein a beam diameter of the laser beam passed through the cylindrical lens in a second direction perpendicular to the first direction in the plane that includes the beam receiving faces of the first and second photodetectors is shorter than a length of any of the beam receiving faces of the first and second photodetectors in the second direction.

21. A semiconductor laser device according to claim 19, wherein the beam receiving faces of the first and second photodetectors are inclined relative to the plane perpendicular to the optical axis of the semiconductor laser device by an angle larger than a maximum angle formed between the laser beam passed through the cylindrical lens and the optical axis of the semiconductor laser device.

22. A semiconductor laser device according to claim 19, wherein an optical length between the semiconductor laser device and an intersection of the optical axis of the semiconductor laser and a plane that includes beam receiving faces of the first and second photodetectors is expressed by L, an optical length between the optical axis of the semiconductor laser device and a position of the beam receiving faces of the first and second photodetectors farthest from the optical axis of the semiconductor laser device is expressed by D, and the beam receiving faces of the first and second photodetectors are inclined by an angle larger than tan.sup.-1 (D/L) relative to the plane perpendicular to the optical axis of the semiconductor laser device.

23. A semiconductor laser device according to claim 19, wherein a center axis of the cylindrical lens is shifted relative to the optical axis of the semiconductor laser device.

24. A semiconductor laser device according to claim 3, wherein the cylindrical lens has a cut-out surface.

25. A semiconductor laser device according to claim 24, wherein a flat portion of the cut-out surface is oriented substantially parallel to an optical axis of the semiconductor laser device.

26. A semiconductor laser device according to claim 24, further comprising a positioning member that contacts the cut-out surface.

27. A semiconductor laser device according to claim 24, wherein the laser beam enters a first cylindrical surface portion of the cylindrical lens adjacent to a first edge of the cut-out surface and exits a second cylindrical surface portion of the cylindrical lens adjacent to a second edge of the cut-out surface.

28. A semiconductor laser device according to claim 27, wherein the first and second cylindrical surface portions each have an antireflection coating.

29. A semiconductor laser device according to claim 3, further comprising an L-shaped positioning member that contacts a cylindrical surface of the cylindrical lens at two locations on the cylindrical lens.

30. A semiconductor laser device according to claim 29, wherein each of the first and second photodetectors has an elongated beam receiving face and wherein an elongation direction of each of the first and second photodetectors is oriented perpendicular to a center axis of the cylindrical lens.

31. A semiconductor laser device according to claim 30, wherein a beam diameter of the laser beam passed through the cylindrical lens in a first direction parallel to the center axis of the cylindrical lens in a plane that includes a beam receiving face of the first photodetector and a beam receiving face of the second photodetector is longer than a summed length of both the beam receiving face of the first photodetector and the beam receiving face of the second photodetector in the first direction, and wherein a beam diameter of the laser beam passed through the cylindrical lens in a second direction perpendicular to the first direction in the plane that includes the beam receiving faces of the first and second photodetectors is shorter than a length of any of the beam receiving faces of the first and second photodetectors in the second direction.

32. A semiconductor laser device according to claim 31, wherein the cylindrical lens has a cut-out surface and wherein a flat portion of the cut-out surface is oriented substantially parallel to an optical axis of the semiconductor laser device.

33. A semiconductor laser device according to claim 3, further comprising: a package wherein the semiconductor laser, the first and second photodetectors and the wavelength filter are housed therein; and a wedge-shaped window attached to the package and having a wedge-shaped cross section, wherein the semiconductor laser is configured to further emit another laser beam which is transmitted outside the package through the wedge-shaped window.

34. A semiconductor laser device according to claim 33, further comprising: another lens which is arranged between the semiconductor laser and the wedge-shaped window and which is separated from the wedge-shaped window such that the another lens does not receive a reflected portion of the another laser beam reflected by the wedge-shaped window.

35. A semiconductor laser device according to claim 33, wherein the wedge-shaped window has an inclined surface which is inclined relative to a direction perpendicular to an optical axis of the semiconductor laser device.

36. A semiconductor laser device according to claim 35, wherein the package has a bottom portion supporting the semiconductor laser and wherein the inclined surface faces the bottom portion of the package.

37. A semiconductor laser device, comprising: a semiconductor laser configured to emit a laser beam; a cylindrical lens configured to allow a laser beam emitted from the semiconductor laser to pass therethrough; detecting means for detecting the laser beam passed through the cylindrical lens; and intensity changing means for changing the intensity of a portion of the laser beam depending upon the wavelength of the laser beam, the intensity changing means being disposed in an optical path between the semiconductor laser and the detecting means such that another portion of the laser beam is detected by the detecting means without impinging upon the intensity changing means.

38. A semiconductor laser device according to claim 3, wherein the beam receiving face of the first photodetector and the beam receiving face of the second photodetector are placed on different planes from each other.

39. A method of monitoring the wavelength of a laser beam emitted by a semiconductor laser, comprising: directing a laser beam through a cylindrical lens, thereby forming a uniaxially converged laser beam; directing a first portion of the uniaxially converged laser beam through a wavelength filter to a first photodetector; directing a second portion of the uniaxially converged laser beam to a second photodetector; determining a signal intensity ratio of a first signal intensity measured by the first photodiode to a second signal intensity measured by the second photodiode; and comparing the signal intensity ratio to a reference signal intensity ratio that corresponds to a present wavelength.

40. The method of claim 39, wherein the second portion of the uniaxially converged laser beam is directed to the second photodetector without passing through the wavelength filer.

Description

The present application is based upon Japanese Patent Application No. 2000-371471 filed Dec. 6, 2000 and Japanese Patent Application No. 2001-132746 filed Apr. 27, 2001, the entire disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength monitor and a semiconductor laser device in which a wavelength of a laser beam outputted from a semiconductor laser is monitored.

2. Description of Related Art

A dense wavelength division multiplexing (DWDM) optical communication has been performed in an optical communication field using optical fibers. In this DWDM optical communication, laser beams, which are emitted from a number of semiconductor lasers and have various wavelengths, pass through a plurality of optical fibers and are multiplexed to produce a multiplexed laser beam, the multiplexed laser beam is lead to an optical fiber, and the multiplexed laser beam is transmitted to a destination. Thereafter, the multiplexed laser beam is demultiplexed to a plurality of laser beams, and the laser beams are used for various purposes.

In recent years, a technique in which laser beams are multiplexed at high density has been proposed in order to efficiently transmit the laser beams. In this technique, wavelength intervals of the laser beams to be multiplexed are narrowed (for example, wavelength intervals of the laser beams are set to specific wavelength intervals equivalent to 50 GHz). Therefore, to multiplex the laser beams without interfering with each other, it is required for each semiconductor laser device to set a wavelength of the laser beam with high stability. To achieve this requirement, an intensity and wavelength of a backward-directed laser beam (also referred to as a backward laser beam), which is emitted from a semiconductor laser simultaneously with a forward-directed laser beam (also referred to as a forward laser beam) to an optical fiber, is detected and monitored, and the wavelength of the backward laser beam is controlled according to the intensity of the backward laser beam to adjust an wavelength of the forward laser beam. Also, in a laser beam measuring field, an intensity and wavelength of a backward laser beam emitted from a semiconductor laser is monitored, and a wavelength of a beam of homogeneous light emitted from the semiconductor laser is measured with high precision.

FIG. 23 is a view schematically showing the configuration of a conventional wavelength monitor in which an intensity and varying wavelength of a backward laser beam emitted from a semiconductor laser is monitored. The conventional wavelength monitor shown in FIG. 23 is disclosed in Published Unexamined Japanese Patent Application H10-79551 (1998). As shown in FIG. 23, a backward laser beam emitted from a semiconductor laser 126 is collimated in a lens 127, and the collimated laser beam transmits through a quarter wavelength plate 128 to transform a linear polarization of the laser beam into a circular polarization. Thereafter, the circularly polarized laser beam is incident on a first polarized beam splitter 129 to divide the incident laser beam into a first laser beam 130 and a second laser beam 131. The first polarized beam splitter 129 has a band pass filter film 132 placed on a first output end face. The first laser beam 130 transmits through the band pass filter film 132 and is received in a first photodiode 133. An output of electric current of the first laser beam 130 detected in the first photodiode 133 fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser 126. The second laser beam 131 is incident on a second polarized beam splitter 134 to divide the incident laser beam into a third laser beam 135 and a fourth laser beam 136. The second polarized beam splitter 134 has a band pass filter film 137 placed on a third output end face. The third laser beam 135 transmits through a band pass filter film 137 and is received in a second photodiode 138. An output of electric current of the third laser beam 135 detected in the second photodiode 138 fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser 126. The fourth laser beam 136 is received in a third photodiode 139 to detect an output of electric current of the fourth laser beam 136. In the conventional wavelength monitor, the outputs of electric current detected in both the first photodiode 133 and the second photodiode 138 are used to monitor the wavelength of the backward laser beam emitted from the semiconductor laser 126, and the output of electric current detected in the third photodiode 139 is used to monitor the intensity of the backward laser beam emitted from the semiconductor laser 126. Therefore, the wavelength and intensity of a forward laser beam emitted from the semiconductor laser 126 can be stabilized.

However, because the conventional wavelength monitor has the above-described configuration, two polarized beam splitters 129 and 134 and two band pass filters 132 and 137 are required. Therefore, a problem has arisen that the number of parts is increased in the conventional wavelength monitor so as to heighten a product cost.

Also, because the backward laser beam emitted from the semiconductor laser 126 is split to propagate in three directions, optical elements such as the lens 127 adjusting the convergence of the backward laser beam emitted from the semiconductor laser 126, the polarized beam splitters 129 and 134 and the photodiodes 133, 138 and 139 are widely separated in a plane. In this case, another problem has arisen that it is difficult to accurately arrange the optical elements with respect to the backward laser beam propagated in three directions.

Also, because three plates, on which the photodiodes 133, 138 and 139 are arranged respectively, separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time, a positional relationship among the semiconductor laser 126, the lens 127 and the photodiodes 133, 138 and 139 undergoes variation. In this case, another problem has arisen that an intensity of the laser beam detected in each photodiode fluctuates even though an intensity of the laser beam emitted from the semiconductor laser 126 is constant.

Also, because the second and third photodiodes 138 and 139 are arranged on two planes positioned orthogonal to each other, the planes separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time. Therefore, another problem has arisen that an output of electric current of the laser beam detected in each photodiode is not stabilized.

In Published Unexamined Japanese Patent Application H5-149793 (1993), a conventional semiconductor laser device is disclosed. In this device, a semiconductor laser and a wavelength monitor for detecting a varying wavelength of a laser beam emitted from the semiconductor laser are arranged. Also, in Published Unexamined Japanese Patent Application S58-12831 (1983), a wavelength measuring device for detecting a wavelength of a laser beam is disclosed. In these devices, a laser beam emitted from a beam source is directly received by one photodetector. Also, the laser beam is received by another photodetector through a filter. These photodetectors are placed on a carrier. In this case, though the precision of the position of the photodetectors is relatively high with respect to a vertical direction, it is difficult to precisely arrange the photodetectors in horizontal directions. Therefore, a problem has arisen that it is difficult to precisely arrange the photodetectors in horizontal directions such that the whole laser beam is correctly detected in the photodetectors or such that a preset part of the laser beam is correctly detected in each photodetector. Also, positions of the photodetectors shift from each other in horizontal directions due to a temperature variation and/or a mechanical variation occurring over a long period of time. Therefore, another problem has arisen that it is difficult to stably and correctly detect a wavelength of the laser beam.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, with due consideration to the drawbacks of the conventional wavelength monitor and the conventional semiconductor laser device, a wavelength monitor and a semiconductor laser device in which a wavelength of a laser beam emitted from a semiconductor laser is always monitored with high precision.

Also, another object of the present invention is to provide a wavelength monitor and a semiconductor laser device in which an intensity and varying wavelength of a laser beam emitted from a semiconductor laser is correctly monitored by using a simple arrangement of constituent elements that does not require a plurality of polarized beam splitters and a plurality of band pass filters.

According to one aspect of the present invention, a wavelength monitor is provided comprising a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam passed through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.

According to another aspect of the present invention, a semiconductor laser device is provided comprising a semiconductor laser configured to emit a laser beam, a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam passed through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.

In the above configurations, the laser beam transmitted through the cylindrical lens is uniaxially converged in a convergence direction, and the uniaxially converged laser beam is received by the first and second photodetectors. Therefore, even if the position of an optical element such as the semiconductor laser, the first photodetector or the second photodetector is shifted in the convergence direction, the laser beam is still received by the first and second photodetectors. Also, even if the position of an optical element is shifted in a direction perpendicular to the convergence direction in a plane of beam receiving faces of the first and second photodetectors, a beam area of the laser beam received by each photodetector does not change.

Accordingly, the wavelength of the laser beam emitted from the semiconductor laser can be always monitored with high precision according to both portions of the laser beam received by the first and second photodetectors, respectively, regardless of whether the position of the semiconductor laser, the first photodetector or the second photodetector is shifted.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description of exemplary embodiments, in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side view showing an optical system including a semiconductor laser device according to a first embodiment of the present invention;

FIG. 2 shows a relationship between the output of electric current and the wavelength of a laser beam;

FIG. 3 is a block diagram of a wavelength division multiplexing type semiconductor laser device;

FIG. 4 shows a plan view of two beam receiving faces of a monolithic photodiode according to a second embodiment;

FIG. 5 shows a plan view of beam receiving faces of four photodiodes of a monolithic photodiode device according to a third embodiment;

FIG. 6 shows a plan view of two laterally-lengthened beam receiving faces of two photodiodes of a monolithic photodiode device according to a fourth embodiment;

FIG. 7 shows a plan view of semicircular beam receiving faces of two photodiodes of a monolithic photodiode device according to a fifth embodiment;

FIG. 8 is an upper-oblique view of a wavelength monitor according to a sixth embodiment of the present invention;

FIG. 9 is a block diagram of a wavelength control system of the wavelength monitor shown in FIG. 8;

FIG. 10A is an explanatory view showing a circular backward laser beam received in photodiodes in case of no positional shift according to the first embodiment;

FIG. 10B is an explanatory view showing a circular backward laser beam received in photodiodes in case of the movement of the beam in the X direction due to a positional shift according to the first embodiment;

FIG. 10C is an explanatory view showing a circular backward laser beam received in photodiodes in case of the movement of the beam in the Y direction due to a positional shift according to the first embodiment;

FIG. 11A is an explanatory view showing a longitudinally-lengthened elliptical beam of a signal laser beam wherein there is no positional shift according to the sixth embodiment;

FIG. 11B is an explanatory view showing a longitudinally-lengthened elliptical beam of a signal laser beam wherein there is the movement of the beam in the X direction due to a positional shift according to the sixth embodiment;

FIG. 11C is an explanatory view showing a longitudinally-lengthened elliptical beam of a signal laser beam wherein there is the movement of the beam in the Y direction due to a positional shift according to the sixth embodiment;

FIG. 12A is an explanatory view showing a positioning structure of a drum lens according to a modification of the sixth embodiment;

FIG. 12B is an explanatory view showing a positioning structure of a drum lens according to another modification of the sixth embodiment;

FIG. 13 is an upper-oblique view of a wavelength monitor according to a seventh embodiment of the present invention;

FIG. 14 is a side view of a drum lens in which surfaces are cut out;

FIG. 15 is an upper view of a wavelength monitor according to an eighth embodiment of the present invention;

FIG. 16 is an explanatory view showing both an incident angle and a reflection angle of a laser beam incident on photodiodes shown in FIG. 15;

FIG. 17 is a side view of a wavelength monitor according to a ninth embodiment of the present invention;

FIG. 18 is an explanatory view showing both an incident angle and a reflection angle of a laser beam incident on photodiodes shown in FIG. 17;

FIG. 19 is a top view of a wavelength monitor according to a tenth embodiment of the present invention;

FIG. 20 is an upper-oblique view of a wavelength monitor according to an eleventh embodiment of the present invention;

FIG. 21 is a side view of a semiconductor laser device according to a twelfth embodiment of the present invention;

FIG. 22 is an upper-oblique view of a wavelength monitor according to a thirteenth embodiment of the present invention; and

FIG. 23 is a view schematically showing an optical system of a conventional wavelength monitor in which an intensity and varying wavelength of a laser beam is monitored.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described with reference to the accompanying drawings.

Embodiment 1

FIG. 1 is a side view showing an optical system including an exemplary semiconductor laser device according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 indicates a semiconductor laser. A forward laser beam (not shown) is emitted from the semiconductor laser 1 in a forward direction (the left direction in FIG. 1), and a signal laser beam 2 (also referred to as a backward laser beam) is emitted from the semiconductor laser 1 in a rearward direction (the right direction in FIG. 1). Reference numeral 3 indicates a lens for collimating the signal laser beam 2 emitted from the semiconductor laser 1. Reference numeral 4 indicates a wavelength filter for receiving a lower half part of the signal laser beam 2 and for transmitting the signal laser beam 2 according to transmissivity of the wavelength filter 4 that varies according to the wavelength of the signal laser beam 2. Reference numeral 5 indicates a first photodiode (or a first photodetector) for detecting an intensity of the signal laser beam 2 transmitting through the wavelength filter 4. Reference numeral 6 indicates a second photodiode (or a second photodetector) for directly detecting an intensity of the signal laser beam 2 collimated by the lens 3. The lens 3, the wavelength filter 4, the first photodiode 5 and the second photodiode 6 can be arranged on a base carrier 11, and the semiconductor laser 1 can be mounted on a chip carrier 50 placed on the base carrier 11. Also, reference numeral 9 indicates a thermister (or a temperature detecting unit) that can be included and can be arranged on the base carrier 11 in the vicinity of the semiconductor laser 1. The base carrier 11 can be arranged on a constant-temperature-keeping device 10 (for example, a Peltier device). The semiconductor laser device 100 shown in FIG. 1 can be arranged in a package (not shown).

In FIG. 1, a propagation direction of the signal laser beam 2 is defined as a Z direction (the right direction on a horizontal plane in FIG. 1), a semiconductor laser mounting direction (the upper direction perpendicular to the horizontal plane) directed from the chip carrier 50 to the semiconductor laser 1 is defined as a Y direction, and a direction (directed perpendicular out of the page) perpendicular to the Y and Z directions is defined as an X direction. In the semiconductor laser 1, two cladding layers (not shown) and an active layer (not shown) placed between the cladding layers are stacked in the Y direction.

Next, an exemplary operation of the semiconductor laser device will be described. A spreading angle of the signal laser beam 2 emitted from the semiconductor laser 1 is adjusted in the lens 3 to collimate the signal laser beam 2. Thereafter, a lower half part of the signal laser beam 2 transmits through the wavelength filter 4 and is received by the first photodiode 5. Also, the other upper half part of the signal laser beam 2 transmitting through the lens 3 is directly received in the second photodiode 6 without passing through the wavelength filter 4. Because the wavelength filter 4 has a transmissivity that depends on the wavelength of an incident laser beam, the intensity of the signal laser beam 2 transmitted through the wavelength filter 4 varies according to the wavelength of the signal laser beam 2. Therefore, an output of electric current in the first photodiode 5 changes with the wavelength of the signal laser beam 2. In this embodiment, the output of electric current in the first photodiode 5 is used as a wavelength monitored output.

Also, because the second photodiode 6 directly receives the signal laser beam 2 collimated by the lens 3 without passing through the wavelength filter 4, the output of electric current in the second photodiode 6 is not changed in dependence on the wavelength of the signal laser beam 2, but is changed with the intensity of the signal laser beam 2 emitted from the semiconductor laser 1. Therefore, the output of electric current in the second photodiode 6 is used as an intensity monitored output.

Also, because the signal laser beam 2 is collimated by the lens 3, even if a position of photodiode 5 or 6 is shifted from an optical axis of the optical system, the signal laser beam 2 transmitting through the wavelength filter 4 will not be received by the second photodiode 6. Therefore, a distance between the first and second photodiodes 5 and 6 can be shortened. In this embodiment, two beam receiving faces of the first and second photodiodes 5 and 6 are arranged on a photodiode substrate so as to be adjacent to each other.

A Fabry-Perot resonator, for example, can be used as the wavelength filter 4. The Fabry-Perot resonator can be formed by depositing a mirror on a surface of a glass substrate polished so as to make the mirror parallel to the surface of the glass substrate. FIG. 2 shows an exemplary relationship between the output of electric current from the first photodiode 5 and the wavelength of the laser beam emitted from the semiconductor laser 1 in cases where a Fabry-Perot resonator is used as the wavelength filter 4. In FIG. 2, a solid line 7 indicates a waveform of the output of electric current generated by the first photodiode 5, and a dotted line 8 indicates the output of electric current generated by the second photodiode 6.

As shown in FIG. 2, the output of electric current from photodiode 5 due to the laser beam transmitting through the Fabry-Perot resonator is periodically increased or decreased with the wavelength of beam incident on the Fabry-Perot resonator. For example, in cases where a wavelength of a laser beam incident on the Fabry-Perot resonator is lengthened within a specific wavelength band B, the output of electric current generated by the first photodiode 5 is sharply decreased. Therefore, in cases where the semiconductor laser 1 emits a laser beam for which the variation in wavelength corresponds to the specific wavelength band B, variations in wavelength of the laser beam 2 emitted from the semiconductor laser 1 can be precisely detected and monitored because the output of electric current obtained in the first photodiode 5 sharply changes with the wavelength of the laser beam 2.

In this embodiment, the specific wavelength band B can be set such that the output of electric current generated by the first photodiode 5 decreases as the wavelength of the laser beam 2 emitted from the semiconductor laser 1 is increased. Alternatively, the specific wavelength band B can be set such that the output of electric current generated by the first photodiode 5 sharply increases in cases where a wavelength of a laser beam incident on the Fabry-Perot resonator is increased. In addition, a table indicating a relationship between the output of electric current generated by the first photodiode 5 and wavelength of a laser beam can be stored in a memory (not shown). Such a table can be generated by actually measuring the output of electric current generated by the first photodiode 5 in advance while changing the wavelength of a laser beam. In this case, it is possible to measure the wavelength of the laser beam 2 emitted from the semiconductor laser 1 according to the output of electric current obtained in the first photodiode 5 by referring to the table stored in the memory.

In contrast, as shown by the dotted line 8 indicating the output in the second photodiode 6, because the portion of the laser beam 2 not transmitted through the wavelength filter 4 is detected by the second photodiode 6, the output in the second photodiode 6 does not change even if the wavelength of the laser beam 2 emitted from the semiconductor laser 1 changes.

In this embodiment, the wavelength filter 4 is not limited to a Fabry-Perot resonator. Any appropriate filter can be used as the wavelength filter 4 on the condition that an intensity of a laser beam transmitting through the filter is changed in dependence on the wavelength of the laser beam. For example, an optical band pass filter obtained by depositing a plurality of dielectric films on a glass substrate can be used as the wavelength filter 4. Also, a filter composed of a pair of birefringent crystals and a polarizer can be used as the wavelength filter 4.

Also, this embodiment is not limited to a configuration wherein the wavelength filter 4 receives a portion of the laser beam 2. For example, a wavelength filter composed of a first block having a wavelength-dependent transmissivity and a second block having no wavelength-dependent transmissivity can be used. In this case, the whole signal laser beam 2 can be received by the wavelength filter.

Next, a method of controlling the semiconductor laser 1 to stabilize the wavelength of the laser beam emitted from the semiconductor laser 1 is described.

FIG. 3 is a block diagram of an exemplary wavelength division multiplexing (WDM) type semiconductor laser device. A method of controlling the semiconductor laser 1 is performed using a wavelength monitor applied to the WDM type semiconductor laser device. In this type of wavelength monitor, a control operation for setting an intensity of the laser beam 2 to a constant value is performed using an automatic power control (APC) circuit 26, and the temperature of the semiconductor laser 1 is controlled to be set to a constant value using an automatic temperature control (ATC) circuit 27. In the APC circuit 26, a driving current supplied to the semiconductor laser 1 is adjusted according to the output of electric current generated by the second photodiode 6, so that an intensity of the forward laser beam emitted from the semiconductor laser 1 is set to a constant value. Also, in the ATC circuit 27, a current supplied to the constant-temperature-keeping device 10, such as a Peltier device, is adjusted according to a resistance value of the thermister 9 arranged in the neighborhood of the semiconductor laser 1. The resistance value of the thermister 9 indicates a temperature of the semiconductor laser 1. Of course, an appropriate temperature detecting unit other than a thermister can also be used.

In the WDM type semiconductor laser device using the wavelength monitor, an intensity of the forward laser beam emitted from the semiconductor laser 1 is set to a setting value by the APC circuit 26 according to the output of electric current generated by the second photodiode 6. For example, in cases where the output of electric current generated by the second photodiode 6 is larger than a setting output, a drive current supplied to the semiconductor laser 1 is lowered by the APC circuit 26. In contrast, in cases where the output of electric current generated by the second photodiode 6 is smaller than a setting output, a drive current supplied to the semiconductor laser 1 is increased by the APC circuit 26. Also, the temperature of the semiconductor laser 1 is adjusted to a setting temperature by the ATC circuit 27 according to a resistance value of the thermister 9. Also, the setting temperature in the ATC circuit 27 is further adjusted according to the output of electric current generated by the first photodiode 5 to keep the output of electric current generated by the first photodiode 5 to a setting value. Accordingly, the wavelength of the forward laser beam emitted from the semiconductor laser 1 can be stabilized to a preset wavelength. For example, in cases where the wavelength of the signal laser beam 2 emitted from the semiconductor laser 1 is set within the specific wavelength band B, when the output of electric current generated by the first photodiode 5 is larger than a setting output, it is determined that the wavelength of the signal laser beam 2 is shorter than the preset wavelength. Therefore, the setting temperature in the ATC circuit 27 is increased to lengthen the wavelength of the signal laser beam 2 emitted from the semiconductor laser 1. In contrast, when the output of electric current obtained in the first photodiode 5 is smaller than a setting output, it is determined that the wavelength of the signal laser beam 2 is longer than the preset wavelength. Therefore, the setting temperature in the ATC circuit 27 is decreased to shorten the wavelength of the signal laser beam 2 emitted from the semiconductor laser 1. That is, the output of electric current generated by the first photodiode 5 is fed back to the ATC circuit 27 without inverting the output of electric current obtained in the first photodiode 5.

In the first embodiment shown in the example of FIG. 1, the signal laser beam 2 is divided into the upper half part of the beam not incident on the wavelength filter 4 and the lower half part of the beam incident on the wavelength filter 4 along the Y direction. However, if the alignment between the semiconductor laser 1 and the first and second photodiodes 5 and 6 is performed with high precision, the signal laser beam 2 can be divided into two parts along the X direction. Also, the signal laser beam 2 can be divided into two parts at an arbitrary ratio as long as the output of electric current generated by the first photodiode 5 is appropriately used to monitor the wavelength of the laser beam 2 and as long as the output of electric current generated by the second photodiode 6 is appropriately used to monitor the intensity of the laser beam 2.

Also, in the first embodiment shown in the example of FIG. 1, the signal laser beam 2 emitted from the semiconductor laser 1 is collimated by the lens 3. However, this embodiment is not limited to collimation by the lens 3. The signal laser beam 2 emitted from the semiconductor laser 1 can alternatively either be converged or diverged by the lens 3 as long as the part of the laser beam not transmitted through the wavelength filter 4 is not received by the first photodiode 5 and as long as the part of the laser beam transmitted through the wavelength filter 4 is not received by the second photodiode 6. Also, it is possible for no lens to be arranged in the exemplary semiconductor laser device shown in FIG. 1.

Accordingly, an intensity and varying wavelength of the laser beam emitted from the semiconductor laser 1 can be correctly monitored by arranging the lens 3, the wavelength filter 4 and the first and second photodiodes 5 and 6 along one optical axis. Also, because a combination of a plurality of polarized beam splitters and a plurality of band pass filters is not used, the semiconductor laser device including the wavelength monitor can be easily manufactured, and a manufacturing cost of the semiconductor laser device including the wavelength monitor can be reduced.

Embodiment 2

FIG. 4 shows a plan view of two beam receiving faces of an exemplary monolithic photodiode device 12 according to a second embodiment. As shown in FIG. 4, the monolithic photodiode device 12 can be arranged in place of the first and second photodiodes 5 and 6. The monolithic photodiode device 12 is obtained by forming two beam receiving faces 13 and 14 on a monolithic photodiode substrate.

Accordingly, the arrangement of the first and second photodiodes 5 and 6 can be performed at one time by arranging the monolithic photodiode on the base carrier 11, and a manufacturing cost of the semiconductor laser device including the wavelength monitor can be further reduced.

Embodiment 3

FIG. 5 shows a plan view of beam receiving faces of four photodiodes of an exemplary monolithic photodiode device 15 according to a third embodiment. As shown in FIG. 5, the monolithic photodiode device 15 is arranged in place of the first and second photodiodes 5 and 6. The monolithic photodiode device 15 is obtained by forming beam receiving faces of four photodiodes 16 to 19 on a monolithic photodiode substrate. The photodiodes 18 and 19 functioning as the second photodiode 6 are placed adjacent to photodiodes 16 and 17 functioning as the first photodiode 5. The photodiodes 16 and 17 are arranged adjacent to each other in the X direction, and the photodiodes 18 and 19 are arranged adjacent to each other in the X direction.

In the first embodiment, to sufficiently receive portions of the signal laser beam 2 emitted from the semiconductor laser 1 on both the first and second photodiodes 5 and 6, it is necessary to precisely align the first and second photodiodes 5 and 6 with the semiconductor laser 1 along the optical axis. As shown in FIG. 1, because the semiconductor laser 1, the lens 3 and the first and second photodiodes 5 and 6 are arranged on the base carrier 11 or the chip carrier 50, the arrangement precision of the first and second photodiodes 5 and 6 in the semiconductor laser mounting direction (the Y direction in FIG. 1) is comparatively high. However, the semiconductor laser 1 and the first and second photodiodes 5 and 6 are typically fixed to the base carrier 11 or the chip carrier 50 using solder or adhesive agent. Therefore, when the temperature of the base carrier 11 or the chip carrier 50 is increased during the manufacturing of the semiconductor laser device, there is a possibility that positions of the semiconductor laser 1 and the first and second photodiodes 5 and 6 can lose proper alignment. Therefore, the arrangement precision of the first and second photodiodes 5 and 6 can deteriorate in a direction (the X direction in FIG. 1) perpendicular to the semiconductor laser mounting direction.

To solve this potential problem, in the third embodiment, the beam receiving area of photodiodes, upon which the laser beam emitted from the semiconductor laser 1 is received, is enlarged in the X direction. That is, the signal laser beam 2 transmitting through the wavelength filter 4 is received on beam receiving faces of two photodiodes 16 and 17 functioning as the first photodiode 5, and the signal laser beam 2 not transmitting through the wavelength filter 4 is received on beam receiving faces of two photodiodes 18 and 19 functioning as the second photodiode 6. Because a first group of photodiodes 18 and 19 and a second group of photodiodes 16 and 17 respectively extend in the X direction, even though the signal laser beam 2 emitted from the semiconductor laser 1 is shifted in the X direction, a decrease of the output of electric current generated by each group of photodiodes can be avoided.

Accordingly, in the third embodiment, the precision of the X-directional arrangement of the semiconductor laser 1, the lens 3, the first group of photodiodes 18 and 19 and the second group of photodiodes 16 and 17 can have lower minimum requirement. For example, when a positional relationship among the semiconductor laser 1, the lens 3, the first group of photodiodes 18 and 19 and the second group of photodiodes 16 and 17 is proper in the X direction, the intensity and wavelength of the laser beam emitted from the semiconductor laser 1 is controlled according to the outputs of electric current obtained in the photodiodes 16 and 17 and the outputs of electric current obtained in the photodiodes 18 and 19, respectively. If the positional relationship deteriorates, the intensity and wavelength of the laser beam emitted from the semiconductor laser 1 is controlled according to a sum of the outputs of electric current generated by the photodiodes 16 and 17 and a sum of the outputs of electric current generated by the photodiodes 18 and 19. Therefore, the photodiodes 16 to 19 of the monolithic photodiode device 15 can compensate for potential deterioration of the X-directional precision in the placement of components for the monitoring of the intensity and varying wavelength of the laser beam, and the precision of the X-directional arrangement can have lower minimum requirement.

Also, in cases where the arrangement precision required for the semiconductor laser device is very high, it is preferred that the number of photodiodes arranged adjacent to each other in the X direction is increased. In this case, the precision of the X-directional arrangement can have lower minimum requirement.

In addition, the semiconductor laser device can alternatively be configured such that the signal laser beam 2 is divided in the Y direction by the wavelength filter 4. In this case, the signal laser beam 2 not transmitting through the wavelength filter 4 is received by a first group of the photodiodes 17 and 19 arranged adjacent to each other in the Y direction, and the laser beam 2 transmitting through the wavelength filter 4 is received by a second group of the photodiodes 16 and 18 arranged adjacent to each other in the Y direction. Even if the laser beam 2 emitted from the semiconductor laser 1 is shifted in the Y direction, because the beam receiving faces of each group of the photodiodes extend in the Y direction, the photodiodes 16 to 19 of the monolithic photodiode device 15 can compensate for potential deterioration of the Y-directional precision in the placement of components for the monitoring of the intensity and varying wavelength of the signal laser beam 2, and the precision of the Y-directional arrangement can have a lower minimum requirement. Also, in cases where the Y-directional arrangement precision required for the semiconductor laser device is very high, it is preferred that the number of photodiodes arranged adjacent to each other in the Y direction is increased for each group of photodiodes. In this case, the precision of the Y-directional arrangement can have lower minimum requirement.

Embodiment 4

FIG. 6 shows a plan view of laterally-lengthened beam receiving faces of two photodiodes of an exemplary monolithic photodiode device 20 according to a fourth embodiment. As shown in FIG. 6, the monolithic photodiode device 20 is arranged in place of the first and second photodiodes 5 and 6. The monolithic photodiode device 20 is obtained by forming beam receiving faces of two photodiodes 21 to 22 respectively lengthened in the X direction on a monolithic photodiode substrate. The photodiode 22 functioning as the second photodiode 6 is placed adjacent to the photodiode 21 functioning as the first photodiode 5.

Accordingly, in the fourth embodiment, because the beam receiving faces of the photodiodes 21 to 22 are lengthened in the X direction, even though the laser beam 2 emitted from the semiconductor laser 1 is shifted in the X direction, the beam receiving faces of the photodiodes 21 to 22 laterally-lengthened in the X direction can reliably receive the laser beam 2. Therefore, the beam receiving faces of the photodiodes 21 to 22 of the monolithic photodiode device 20 can compensate for potential deterioration of the X-directional precision in the placement of components for the monitoring of the intensity and varying wavelength of the laser beam, and the precision of the X-directional arrangement can have lower minimum requirement.

Also, in cases where the arrangement precision required for the semiconductor laser device is very high, it is preferred that the beam receiving faces of the photodiodes 21 and 22 are further lengthened in the X direction. In this case, the precision of the X-directional arrangement can have lower minimum requirement.

Also, in cases where the arrangement precision can be satisfied in the X direction, the wavelength filter 4 can be placed on the right side (or the left side) of the optical axis in a plane parallel to the X-Y plane. In this case, a monolithic photodiode device replacing the first and second photodiodes 5 and 6 of the first embodiment has longitudinally-lengthened beam receiving faces of two photodiodes arranged adjacent to each other in the X direction, and the beam receiving face of each photodiode extends in the Y direction. Also, the portion of the laser beam transmitting through the wavelength filter 4 is received by one photodiode placed on the right side (or the left side) in a plane parallel to the X-Y plane, and the portion of the laser beam not transmitting through the wavelength filter 4 is received by the other photodiode placed on the left side (or the right side) in the plane parallel to the X-Y plane. Therefore, even if the laser beam 2 emitted from the semiconductor laser 1 is shifted in the Y direction, because the beam receiving face of each photodiode extends in the Y direction, the photodiodes of the monolithic photodiode device can compensate for potential deterioration of the Y-directional precision in the placement of components for the monitoring of the intensity and varying wavelength of the laser beam, and the arrangement precision of the semiconductor laser 1, the lens 3 and the photodiodes 5 and 6 in the Y direction can have lower minimum requirement. Also, in cases where the arrangement precision required for the semiconductor laser device is very high, it is preferred that the beam receiving face of each photodiode is further lengthened in the Y direction. In this case, the precision of the Y-directional arrangement can have lower minimum requirement.

Embodiment 5

FIG. 7 shows a plan view of semicircular beam receiving faces of two photodiodes of a monolithic photodiode device according to a fifth embodiment. As shown in FIG. 7, a monolithic photodiode device 23 is arranged in place of the first and second photodiodes 5 and 6. The monolithic photodiode device 23 is obtained by forming semicircular beam receiving faces of two photodiodes 24 and 25 on a monolithic photodiode substrate. The photodiode 25 functioning as the second photodiode 6 of the first embodiment is placed adjacent to the photodiode 24 functioning as the first photodiode 5 of the first embodiment. The beam receiving faces of the photodiode have chord edges 24a and 25a extending in the X direction perpendicular to both the optical axis (Z direction) and the photodiode arranging direction (Y direction), and the chord edges 24a and 25b of the photodiodes 24 and 25 face each other. Therefore, a group of the photodiodes 24 and 25 of the monolithic photodiode device 23 is formed approximately in a circular shape. Because the laser beam emitted from the semiconductor laser 1 is formed approximately in a circular shape, the shape of the laser beam approximately matches the shape of the beam receiving faces of the photodiodes 24 and 25. Therefore, the signal laser beam 2 emitted from the semiconductor laser 1 can be efficiently received by the beam receiving faces of the photodiodes 24 and 25 of the monolithic photodiode device 23.

In this embodiment, the beam receiving face of