JP5103416B2 - Wavelength swept light source - Google Patents

Description

  The present invention relates to a technique for suppressing a mode hop that may occur during wavelength sweeping in a Litman single-mode wavelength sweeping light source.

  Light wavelength, which is one of the basic characteristics of light, can be used to measure various physical characteristics. In the past, systems and measuring instruments using the same have been developed in various fields such as optical communication, spectroscopic analysis, surface shape measurement and optical sensing. In particular, high-monochromatic and coherent high-quality light is extremely useful because it enables advanced systems and high-precision measurements.

  Therefore, for example, Patent Document 1 proposes a wavelength swept light source that can output high-quality light. FIG. 37 shows a basic configuration of the light source 1.

  In the light source, the light emitted from the semiconductor laser 2 is collimated by the collimator lens 3 and then enters the diffraction surface 4a of the diffraction grating 4 at a predetermined angle. Of the diffracted light generated on the diffraction surface 4a, the light emitted in the direction orthogonal to the reflection surface 5a of the mirror 5 supported at a position facing the diffraction grating 4 is returned to the diffraction grating 4 by the reverse optical path and returned. The diffracted light generated on the diffractive surface 4a with respect to the reflected light is returned to the semiconductor laser 2 by a reverse optical path.

  The mirror 5 has a structure that rotates about a specific position O with respect to the diffraction grating 4, and has an optical path length from the effective resonance end face 2 a of the semiconductor laser 2 to the reflection surface 5 a of the mirror 5 through the diffraction grating 4. The determined resonator length and the wavelength of light emitted in the direction orthogonal to the reflecting surface 5 a among the diffracted light in the diffraction grating 4 are changed by the rotation of the reflecting surface 5 a of the mirror 5. Thus, an external resonance type wavelength sweeping light source that continuously sweeps the wavelength of the emitted light of the semiconductor laser in a predetermined range is realized.

  Here, the specific position O is at a position where the extended surface H1 of the diffraction surface 4a of the diffraction grating 4 and the extended surface H2 of the effective resonance end surface 2a of the semiconductor laser 2 intersect, and the mirror 5 has an extended surface H3 of the reflecting surface 5a. It is defined to cross a specific position O.

  This light source is generally called a Litman method, and in principle, laser light that oscillates in a single mode can be continuously swept in a predetermined wavelength range without mode hopping.

  However, the actual wavelength sweeping light source disclosed in Patent Document 1 performs wavelength sweeping by rotating a mirror installed on a large and heavy metal block with a motor and a bearing. The sweep cannot be performed.

  In addition, mode hops are likely to occur due to the low axial stability of the rotation mechanism, and the wavelength at which mode hops change each time the wavelength is swept. Complex position control of the parts is required. Such a mode hop is inevitable even if the temperature of the light source and the current injected into the light emitting region of the semiconductor laser are controlled to be constant.

  In the case of the optical arrangement described above, the mirror or the support member that supports the mirror and rotates integrally with the optical axis intersects the mirror or the support member with a hole or notch for allowing light to pass through. In some cases, the mirror or its supporting member is easily deformed due to a reduction in rigidity, which may cause mode hops.

  In order to control this, very complicated control of adjusting the optical axis by finely adjusting the arrangement of the collimating lens and the diffraction grating is necessary, which makes it difficult to quickly sweep the wavelength.

  Therefore, for example, Patent Document 2 proposes a wavelength swept light source that enables a small and fast wavelength sweep using a more stable rotation mechanism. The wavelength sweeping mechanism used in this light source has a so-called MEMS structure that is accurately and compactly formed by etching processing on a semiconductor substrate, etc., and a rectangular reflector is concentrically arranged inside a rectangular frame, Between the middle part of the upper inner edge of the frame and the middle part of the upper edge of the reflector, and between the middle part of the lower edge of the frame and the middle part of the lower edge of the reflector, the torsionally deformable coupling parts are connected. By periodically applying an electrostatic or magnetic force to one or both ends of the reflecting plate, the reflecting plate can be reciprocated by torsional deformation of the connecting portion.

  In the case of this mechanism, the movement of the reflecting plate rotated for the wavelength sweep is extremely stable. Therefore, a Litman-type external resonator type wavelength sweep light source using this mechanism as a mirror part is smaller than before. Thus, a stable high-speed wavelength sweep without single mode and mode hop is enabled.

US Pat. No. 5,319,668

JP 2005-284125 A

  However, even a wavelength swept light source using such a MEMS-structured rotating mirror may cause mode hops due to poor assembly accuracy of the light source or due to optical axis misalignment of the optical component due to temperature changes. .

  Further, when the current injected into the light emitting region and the temperature of the semiconductor laser are adjusted in order to change the power of the output light from the light source, there is a problem that a mode hop inevitably occurs due to the characteristics of the semiconductor laser.

  The present invention solves these problems, and provides a wavelength-swept light source of a Littman-type external resonator type that can be swept continuously and at high speed in a small size and without mode hopping. The purpose is that.

In order to achieve the above object, the wavelength swept light source according to claim 1 of the present invention comprises:
A base (21);
A semiconductor laser (22) fixed on the base and having at least one light emitting end face having a low reflectivity surface;
A collimator lens (23) fixed on the base and converting the light emitted from the low reflectance surface of the semiconductor laser into parallel light;
A predetermined diffraction surface (25b) in which grooves for diffracting light are formed in parallel, and light emitted from the collimating lens is orthogonal to the grooves and non-orthogonal to the diffraction surface A diffraction grating (25) fixed on the base in a state of being incident at a predetermined incident position at an incident angle;
The collimating lens has a reflecting surface facing the diffraction surface of the diffraction grating, and is rotatable in a plane that is parallel to the groove of the diffraction grating and that is orthogonal to the diffraction surface about an axis at a specific position. Of the diffracted light with respect to the light emitted from and incident on the diffraction surface of the diffraction grating, the light along the optical axis perpendicular to the reflection surface is reflected and returned to the diffraction grating by a reverse optical path, and the returned light is incident A rotating mirror (30) for returning to the semiconductor laser via the collimating lens with the same optical axis as the optical path;
A distance r from the rotation center of the rotation mirror to the predetermined incident position of the diffraction grating, a distance L2 from the rotation center to a plane obtained by extending the reflection surface, and the diffraction grating from the effective resonance end surface of the semiconductor laser Between the optical path length L1 to the predetermined incident position and the light incident angle α to the diffraction surface of the diffraction grating,
r = (L1-L2) / sin α
Establishing the relationship
In response to a change in the angle of the reflecting surface of the rotating mirror, the resonator length from the semiconductor laser to the reflecting surface of the rotating mirror via the collimating lens and the diffraction surface of the diffraction grating is changed. In the wavelength sweep light source of the Litman method external resonator type that continuously changes the wavelength of the emitted light within a predetermined range,
The semiconductor laser is
An n-type semiconductor substrate (221), an active layer (223) formed on the n-type semiconductor substrate, and a first p-type cladding layer (224) stacked on the active layer, and for current injection A light emitting region (222) that receives and emits light and guides the light along the active layer;
A waveguide layer (231) having an energy gap larger than the energy of the light on the n-type semiconductor substrate and optically coupled to the active layer of the light emitting region, and laminated on the waveguide layer The second p-type cladding layer (232) has a structure including a phase adjustment region (230) in which a refractive index with respect to light from the light emitting region changes according to a forward injection current or a reverse bias voltage,
The rotating mirror is
A frame (31) fixed to the base; a reflector (32) disposed on the inner side of the frame and having the reflecting surface formed on one side; and an outer edge of the reflector and an inner edge of the frame And a pair of connecting portions (33, 34) that are twistable and connect to each other and are aligned in a straight line parallel to the grooves of the diffraction grating, and periodically apply a force to the end of the reflecting plate. With the dynamic drive means (35, 36, 40), the reflector is reciprocally rotated around the connecting part to sweep the wavelength, and even when the mode hop is generated during the wavelength sweep, the mode hop is highly reproduced. It has a MEMS structure with rotational stability that can be generated at the same wavelength .
further,
Either a light fixed on the base and emitted from an end surface opposite to the one end surface of the semiconductor laser, or a zero-order diffracted light with respect to light incident on the diffraction surface of the diffraction grating from the semiconductor laser A light receiver (60) for receiving and outputting an electrical signal corresponding to the intensity of the received light;
While the output wavelength of the semiconductor laser is swept by the reciprocating rotation of the reflecting plate of the rotating mirror, the output of the light receiver is monitored, and a discontinuous change in the output signal of the light receiver caused by a mode hop is observed. Mode hop detection means (61) for detecting;
Phase adjustment region control means (65) for changing the forward injection current or the reverse bias voltage of the phase adjustment region of the semiconductor laser so as to eliminate the discontinuous change in the output signal of the light receiver detected by the mode hop detection means. ) To change the forward injection current or reverse bias voltage by a predetermined amount, repeat the process of sweeping the next wavelength and detecting the mode hop, so that the mode hop does not occur or the mode hop occurs the least There is provided a phase adjustment region control means for performing a semi-fixed control process for finding a certain forward injection current or reverse bias voltage and holding the constant forward injection current or reverse bias voltage .

Moreover, the wavelength sweep light source of Claim 2 of this invention is the wavelength sweep light source of Claim 1,
The doping concentration in the thickness direction of the second p-type cladding layer in contact with the waveguide layer in the phase adjustment region of the semiconductor laser has a maximum value within a predetermined distance from the upper end of the waveguide layer. It is characterized by that.

Moreover, the wavelength sweep light source of Claim 3 of this invention is the wavelength sweep light source of Claim 2,
The predetermined distance range is 25 to 150 nm.

Moreover, the wavelength sweep light source of Claim 4 of this invention is the wavelength sweep light source in any one of Claims 1-3,
A third p-type cladding layer (240) is formed on the first p-type cladding layer in the light emitting region of the semiconductor laser and the second p-type cladding layer in the phase adjustment region, and the light emitting region of the third p-type cladding layer A separation groove (251, 252) having a predetermined width is formed at a boundary portion with the phase adjustment region.

Moreover, the wavelength sweep light source of Claim 5 of this invention is the wavelength sweep light source of Claim 4,
The doping concentration at the end point in the thickness direction of the third p-type cladding layer is larger than the doping concentration at the start point in the thickness direction, and 1.0 × 10 ¹⁸ (/ cm ³ ) to 2.5 × 10 ¹⁸ (/ cm ^3). ) Range.

Moreover, the wavelength sweep light source of Claim 6 of this invention is the wavelength sweep light source in any one of Claims 1-5,
The semiconductor laser is formed so that a doping concentration in a thickness direction of the first p-type cladding layer in the light emitting region of the semiconductor laser has a maximum value within a range of 50 to 250 nm from an upper end of the active layer.

Moreover, the wavelength sweep light source of Claim 7 of this invention is a wavelength sweep light source in any one of Claims 1-6,
The crossing angle between the element end face (220a) on the light emitting region side of the semiconductor laser and the optical axis of the light guided through the active layer, or the element end face (220b) on the phase adjustment region side and the waveguide layer At least one of the crossing angles with the optical axis of the guided light is non-orthogonal.

Moreover, the wavelength sweep light source of Claim 8 of this invention is the wavelength sweep light source in any one of Claims 1-7,
When the mode hop remains in the semi-fixed control process, the phase adjustment area control means is synchronized with the reciprocating rotation of the reflector during the subsequent wavelength sweep, and the forward direction of the phase adjustment area of the semiconductor laser A dynamic control process is performed in which the injection current or the reverse bias voltage is variably controlled from the constant value for each remaining mode hop to suppress the occurrence thereof.

Moreover, the wavelength sweep light source of Claim 9 of this invention is the wavelength sweep light source in any one of Claims 1-8,
The semiconductor laser, the collimating lens, the fixed mirror, and the diffraction grating have a structure in which light emitted from the collimating lens is incident on the diffraction grating via a fixed mirror (24) fixed on the base. The optical axis extending from the semiconductor laser to the diffraction grating via the fixed mirror is arranged in a non-crossing state with the reflector of the pivot mirror so that it is disposed on one surface side of the reflector of the pivot mirror. It was made to become.

  As described above, in the wavelength swept light source of the present invention, the operation of the rotating mirror is extremely stable, so that the generation of irregular mode hops is remarkably reduced as compared with the conventional light source. Even if mode hops occur regularly due to steady optical axis misalignment, etc., there is a mode hop detection means that detects discontinuous changes in the output of the optical receiver due to mode hops during wavelength sweeping. Therefore, a mode-hop-free state is easily realized and maintained by regularly controlling the forward injection current or reverse bias voltage to the phase adjustment region of the semiconductor laser so that the discontinuous change is eliminated. it can.

  In addition, the doping concentration in the thickness direction of the second p-type cladding layer in contact with the waveguide layer in the phase adjustment region of the semiconductor laser is maximized within a predetermined distance from the waveguide layer. Electron overflow caused by a small ΔEc between the waveguide layer and the second p-type cladding layer can be suppressed, and thereby the maximum phase adjustment amount can be increased. In particular, by making the maximum value in the range of 25 to 150 nm from the waveguide layer, electrons overflowing from the waveguide layer can be effectively blocked, and the maximum phase adjustment amount can be greatly expanded. The control range for mode hop suppression can be expanded.

  In addition, since a separation groove is provided at the boundary between the light emitting region and the phase adjustment region of the third p-type cladding layer, the separation resistance between the regions can be increased, and the leakage current from the light emitting region to the phase adjustment region is greatly increased. Can be reduced.

Further, the doping concentration at the end point in the thickness direction of the third p-type cladding layer (the side far from the active layer and the waveguide layer) is larger than the doping concentration at the start point in the thickness direction (the side close to the active layer and the waveguide layer), In addition, in the case where it is set in the range of 1.0 × 10 ¹⁸ (/ cm ³ ) to 2.5 × 10 ¹⁸ (/ cm ³ ), the waveguide loss is reduced without increasing the series resistance in the film thickness direction. It becomes possible to do.

  In addition, when the doping concentration in the thickness direction of the first p-type cladding layer in the light emitting region has a maximum value in the range of 50 to 250 nm from the active layer, overflow of injected carriers in the light emitting region is suppressed. Therefore, it is not necessary to pass a wasteful current that does not contribute to light emission, and as a result, high efficiency of the entire device can be realized.

  The semiconductor laser has a structure in which light emitted from the semiconductor laser is incident on the diffraction grating via the fixed mirror, and the semiconductor laser, the fixed mirror, and the diffraction grating are arranged on one surface side of the reflecting plate of the rotating mirror, Since the optical axis from the laser to the diffraction grating via the fixed mirror is not crossed with the reflector of the rotating mirror, there is no decrease in the rigidity of the mirror, enabling more stable wavelength sweeping and phase adjustment. The mode hop can be controlled more easily and more stably by the injection current or reverse bias voltage to the region.

Overall configuration diagram of an embodiment of the present invention Plan view of optical unit of embodiment The exploded perspective view of the principal part of an embodiment The figure which shows the relationship between the drive signal and wavelength change of embodiment Diagram for explaining conditions for continuous wavelength sweeping Figure showing the wavelength vs. optical output characteristics when there is no mode hopping or when there is mode hopping and when there is mode hopping, and the absolute value of the time change Diagram showing the relationship between longitudinal mode and resonator selection characteristics The figure which shows the example of the change pattern of the injection current or the reverse bias voltage when suppressing it by the dynamic control with respect to the phase adjustment area when there is a mode hop The figure which shows the modification of the principal part The figure which shows the modification of the principal part The figure which shows the modification of arrangement | positioning of an optical part. The figure which shows the modification of arrangement | positioning of an optical part. External view of a semiconductor laser suitable for an embodiment of the present invention Plan view of the semiconductor laser of FIG. AA line sectional view of FIG. Doping concentration distribution diagram of p-type cladding layer in phase adjustment region Diagram showing the relationship between the current injected into the phase adjustment region and the amount of phase adjustment Doping concentration distribution diagram of p-type cladding layer in light emitting region Doping concentration distribution diagram of the third p-type cladding layer Another example of doping concentration distribution diagram of the third p-type cladding layer The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. The figure for demonstrating the manufacturing process of the semiconductor laser of FIG. Diagram showing a structural example of a semiconductor laser that crosses the waveguide layer and the element end face non-orthogonally Basic configuration diagram of wavelength-swept light source of Litman external resonator type

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of a wavelength swept light source 20 according to an embodiment of the present invention, FIG. 2 is a plan view of an optical unit, and FIG. 3 shows a structure of a main part of the optical unit.

  As shown in FIG. 1 and FIG. 2, the wavelength swept light source 20 is configured on a base 21 having high steps 21a, 21a ′ and low steps 21b whose upper surfaces are parallel to each other. The part 21a includes a semiconductor laser 22 that emits light parallel to the upper surface of the low step part 21b from the low-reflectance element end face 220a (low reflectivity surface), and converts the light emitted from the semiconductor laser 22 into parallel light. A collimating lens 23 that receives the parallel light emitted from the collimating lens 23 by a reflecting surface 24a perpendicular to the upper surface of the high step portion 21a, and reflects the light toward the diffraction surface 25a of the diffraction grating 25 described later. It is fixed.

  The reflected light of the fixed mirror 24 is incident at a predetermined incident angle α in a non-orthogonal direction at a predetermined incident position of the diffraction surface 25a of the diffraction grating 25 standing vertically to the lower step portion 21b of the base 21. A diffraction groove 25b for diffracting light is provided on the diffraction surface 25a of the diffraction grating 25 in parallel with the direction perpendicular to the low step portion 21b, and is orthogonal to the diffraction groove 25b reflected by the fixed mirror 24. In addition, the parallel light incident on the diffractive surface 25a at an incident angle α is diffracted in a direction corresponding to the wavelength by the diffraction groove 25b. In addition, although it demonstrates on the basis of the upper surface of the base 21 regarding the direction of an optical axis or each member, the position of each part is prescribed | regulated relatively and is not limited to arrangement | positioning of this embodiment.

  The light diffracted by the diffraction grating 25 enters the rotating mirror 30. The rotating mirror 30 reflects the diffracted light emitted from the diffraction grating 25 with respect to the parallel light incident from the fixed mirror 24, and the light of the wavelength component input vertically to the diffraction surface 25a of the diffraction grating 25 through the reverse optical path. Thus, a reflecting surface 32 a for returning to the semiconductor laser 22 is provided.

  By periodically changing the angle of the reflecting surface 32a with respect to the diffractive surface 25a of the diffraction grating 25 within a predetermined angle range, the wavelength of light returned to the semiconductor laser 22 by the reflecting surface 32a of the rotating mirror 30 in the reverse optical path is continuous. Change periodically and periodically, whereby the wavelength of the light emitted from the wavelength sweep light source 20 also changes continuously and periodically.

  As shown in FIGS. 1 to 3, the rotating mirror 30 is formed by etching or the like on a conductive substrate (for example, a silicon substrate). For example, the upper plate 31a and the lower plate are illustrated. A frame 31 formed in a horizontally long rectangular frame shape by 31b and horizontal plates 31c and 31d, and a horizontally long rectangle formed concentrically on the inner side of the frame 31 and having a reflecting surface 32a for reflecting light on at least one surface side The upper plate 31a and the lower plate 31b of the frame 31 extend from the center of the inner edge facing each other to the center of the upper and lower edges of the reflector 32 so as to be aligned in a straight line. The plate 31a, the lower plate 31b, and the reflecting plate 32 are connected to each other, and a pair of connecting portions 33 and 34 for rotating the reflecting plate 32 by twisting deformation are provided.

  The reflecting surface 32a of the reflecting plate 32 can be formed of, for example, a mirror finish on the material surface, vapor deposition of a metal film exhibiting high reflectivity, or a dielectric multilayer film. Further, when the rotating mirror 30 is made of a material exhibiting a high reflectance with respect to the laser light, the surface of the material can be used as a reflecting surface without providing a reflecting film or a reflecting sheet.

  However, when the rotating mirror 30 does not have conductivity, it is necessary to deposit a conductive metal film as a mirror material in order to secure an electrostatic driving force described later.

  The widths and lengths of the connecting portions 33 and 34 are set so that the connecting portions 33 and 34 themselves can be twisted and deformed along the longitudinal direction, and a restoring force is generated to return to the original state by the deformation. ing.

  In addition, on both surfaces of one of the horizontal plates 31c and 31d (here, the horizontal plate 31c) of the frame 31 of the rotating mirror 30, electrode plates 35 and 36 for applying external force to the reflecting plate 32 are provided. It is attached via an insulating spacer 37. The electrode plates 35 and 36 are overlapped with a gap corresponding to the thickness of the spacer 37 formed on both surfaces of one end side (here, the left end side) of the reflection plate 32. Here, the spacer 37 has a vertically long rectangular shape, but the spacer 37 may be formed in a rectangular frame shape that overlaps the frame 31 in order to reinforce the entire frame 31.

  In the rotating mirror 30, the rotation center of the reflecting plate 32 (the line connecting the centers of the coupling portions 33 and 34) is in a state where the diffraction surface 25a of the diffraction grating 25 is extended and parallel to the diffraction groove 25b. Thus, it is fixed on the base 21.

  The rotating mirror 30 includes the electrode plates 35 and 36 and the spacers 37 and is formed by etching the silicon substrate as described above, but the manufacturing method is arbitrary.

  For example, the frame 31, the reflecting plate 32, and the connecting portions 33 and 34 of the rotating mirror 30 are formed by etching with a single layer substrate, and the spacers 37 and 37 and the electrode plates 35 and 36 are formed with another substrate. A method of bonding and using a three-layer substrate such as an SOI substrate, the frame 31, the reflector 32, and the connecting portions 33 and 34 of the rotating mirror 30 are etched on the upper substrate, and the lower substrate is formed on the lower substrate. It can be manufactured by various methods such as etching the spacer 37 and attaching the electrode plate 35 manufactured in another process to form one electrode.

  As shown in FIG. 2, the mirror driving device 40 shown in FIG. 1 has, for example, (a) and (b) in FIG. 4 with respect to the two electrode plates 35 and 36 with the frame 31 of the rotating mirror 30 as a reference potential. The signals V1 and V2 that are 180 degrees out of phase as shown in FIG. 6 are applied to alternately generate an electrostatic attractive force between the electrode plates 35 and 36 and the end of the reflecting plate 32. Turn back and forth.

  The frequency of the signals V1 and V2 is set to be equal to the natural frequency of the reflector 32 determined by the shape and weight of the reflector 32 of the rotating mirror 30 and the torsion spring constant of the coupling portions 33 and 34. Therefore, the reflecting plate 32 can be reciprocated at a large angle with a small driving power.

  The reciprocating rotation of the reflecting plate 32 changes the resonator length of the external resonator and the angle of the reflecting surface 32a with respect to the diffractive surface 25a, and the wavelength of the laser light emitted from the semiconductor laser 22 is as shown in FIG. It changes continuously and periodically.

  However, in the case of a simplified structure in which the reflecting plate 32 itself having the reflecting surface 32a formed on one surface side is rotated like the wavelength swept light source 20, the rotation center is the connecting portions 33, 34. Is not on the line connecting the centers of the two, i.e., on the inside of the reflecting plate 32 and extending the reflecting surface 32a, and does not satisfy the conditions of the conventional Litman method.

  Therefore, in the wavelength swept light source 20 of this embodiment, the wavelength is continuously varied by applying the technique disclosed in the following Patent Document 3.

Japanese Patent No. 3069643

That is, in Patent Document 3, a plane in which the diffraction surface 25a of the diffraction grating 25 is extended in a virtual arrangement in which laser light is transmitted through the reflection plate 32 without using the fixed mirror 24 as indicated by the dotted line in FIG. H1 is a plane obtained by extending the effective resonance end face 22a in consideration of the refractive index inside the semiconductor laser 22, and H3 is a plane obtained by extending the reflection surface 32a of the reflection plate 32. The rotation center of the reflection surface 32a and the diffraction grating When the plane H1 and the plane H3 intersect with each other, the distance from the rotation center O to the predetermined incident position G of the diffraction grating 25 is r, and the distance from the predetermined incident position G to the effective resonance end face 22a of the semiconductor laser 22 is When the effective optical path length is L1, the distance from the rotation center O to the plane H3 is L2, and the incident angle α of light with respect to the diffraction grating 25,
r = (L1-L2) / sin α
Thus, the wavelength can be continuously varied without generating a mode hop.

  However, when the optical path from the semiconductor laser 22 to the diffraction grating 25 is bent via the fixed mirror 24 as in this embodiment, the effective optical path length L1 from the predetermined incident position point G to the effective resonance end face 22a of the semiconductor laser 22 is The optical path length L3 from the effective resonance end face 22a of the semiconductor laser 22 to the fixed mirror 24 and the optical path length L4 from the fixed mirror 24 to the predetermined incident position G are expressed as L1 = L3 + L4.

  Therefore, by arranging each part so that the following equation is established, continuous wavelength sweep without mode hopping is possible as shown in FIG.

  r = (L3 + L4-L2) / sin α

  Further, light is incident on the diffraction grating 25 through the fixed mirror 24, and the semiconductor laser 22, the collimating lens 23, the fixed mirror 24, and the diffraction grating 25 are arranged on one surface side of the reflection plate 32, and the reflection plate 32 and the optical path are arranged. Therefore, it is not necessary to provide a hole for passing light through the reflecting plate 32, and deformation due to a decrease in rigidity does not occur, and even a thin plate can perform high-speed wavelength sweeping that is stable and highly reproducible.

  As described above, in the wavelength swept light source 20 of the embodiment, the reflection plate 32 is extended in the space between the plane extending the reflection surface 32a and the plane extending the diffraction surface 25a in the direction of the rotation center, and the rotation center and diffraction. The fixed mirror 24 is arranged between the surface 25a and a predetermined incident position, and the diffraction grating 25 is included in two spaces separated by a surface obtained by extending the reflecting surface 32a of the reflecting plate 32 of the rotating mirror 30. A semiconductor laser 22 and a collimator lens 23 are arranged in the opposite space, light enters the fixed mirror 24 from the semiconductor laser 22 via the collimator lens 23, and the reflected light is incident on a predetermined incident position of the diffraction surface 25a of the diffraction grating 25. Is incident.

  Therefore, regardless of the optical path, the rotating mirror can be configured with a very simple structure in which the reflecting plate 32 itself having the reflecting surface 32a is reciprocally rotated, and the wavelength can be tuned at high speed and with high accuracy.

  However, as described above, even in the case of the wavelength swept light source 20 using the rotating mirror 30 having the MEMS structure, the mode hop is caused by the reason that the assembly accuracy of each part is bad or the optical axis shift of the optical component due to the temperature change. Will occur.

  Further, if the injection current and temperature to the light emitting region of the semiconductor laser are adjusted in order to change the power of the output light, there is a problem that mode hops inevitably occur.

  Therefore, in this embodiment, as shown in FIG. 1, as the semiconductor laser 22, a light emitting region 222 that emits light upon receiving an injection of a current I and guides the light along an internal active layer; A phase adjustment in which a waveguide layer optically coupled to the active layer of the light emitting region 222 is included, and the refractive index with respect to the light from the light emitting region 222 changes according to the forward injection current I ′ or the reverse bias voltage Vr. A structure including the region 230 is used.

  A general structure of the semiconductor laser 22 having the phase adjustment region 230 includes an n-type semiconductor substrate, an active layer formed on the n-type semiconductor substrate, and a first p-type stacked on the active layer. A light emitting region having a clad layer and guiding light generated inside by a predetermined injection current along the active layer; and energy larger than energy of light generated by the light emitting region on the n-type semiconductor substrate A waveguide layer having a gap and optically coupled to the active layer in the light emitting region, and a second p-type cladding layer stacked on the waveguide layer, and having a refractive index with respect to light from the light emitting region in order. It has a structure including a phase adjustment region that changes in accordance with the direction injection current or the reverse bias voltage. A more preferable structural example in which the control range for mode hop suppression is expanded from that of a general semiconductor laser will be described later.

  In the case of the semiconductor laser 22 having the phase adjustment region 230 as described above, a plasma effect is known in which the refractive index decreases when carriers are injected into the phase adjustment region. When the refractive index decreases, the external resonator length becomes shorter and the optical wavelength changes to negative (short wavelength). Therefore, the phase can be adjusted by controlling the carrier injection, that is, the forward injection current. .

  The refractive index can also be changed by applying a reverse bias voltage Vr to the phase adjustment region 230 and changing it. In this case, when the absolute value of the reverse bias voltage Vr is increased, the refractive index increases, the external resonator length becomes longer, and the optical wavelength changes to positive (long wavelength).

  Therefore, phase control can be performed in a wider range by using both the variable forward injection current and the variable reverse bias voltage Vr.

  Further, in order to suppress the mode hop, the 0th order diffracted light emitted in the direction symmetrical to the incident angle α with respect to the light incident on the diffraction grating 25 is received, and an electric signal having a magnitude corresponding to the intensity of the light. In addition to the mirror driving device 40 described above, the mode hop detection means 61 and the light emitting region 222 of the semiconductor laser 22 are included in the control unit 100. The light emitting region current injecting means 62 for injecting a desired current and the forward current I or the reverse bias voltage Vr to the phase adjusting region 230 are selectively given so as to suppress the mode hop, and this is variably controlled. Phase adjustment area control means 65 is provided.

  The phase adjustment region control means 65 includes a forward injection current supply circuit 65 a that supplies a desired forward injection current I ′ to the phase adjustment region 230, and a reverse bias voltage that applies a desired reverse bias voltage Vr to the phase adjustment region 230. A supply circuit 65b is provided to variably control either the forward injection current I 'or the reverse bias voltage Vr so that the mode hop is eliminated.

  In this embodiment, in consideration of emitting wavelength swept light, which is output light as a light source, from the element end face 222b on the phase adjustment region 230 side of the semiconductor laser 22, the 0th-order diffracted light of the diffraction grating 25 is received by the light receiver 60. However, instead of the 0th-order diffracted light, light emitted from the element end face 220b of the semiconductor laser 22 is received by the light receiver 60, and the 0th-order diffracted light is emitted as wavelength swept light that is output light as a light source. There is also.

  The mode hop detection means 61 monitors the output of the light receiver 60 while the wavelength of the emitted light of the semiconductor laser 22 is swept by the reciprocating rotation of the reflecting plate 32 of the rotating mirror 30, and the light receiver 60 generated by the mode hop. Detects discontinuous changes in the output signal.

  Here, for example, consider a case where the light wavelength is swept between 1500 nm and 1600 nm. When no mode hop occurs, the wavelength dependence of the power P of the zero-order light from the diffraction grating 25 detected by the light receiver 60 (change over time at the time of sweep) is continuous as shown in FIG. As a result, the absolute value | dP / dt | of the derivative with respect to time of the output signal of the light receiver 60 also changes gradually. Here, the power P of the light changes during the wavelength sweep because the characteristics of each optical component have wavelength dependency.

  On the other hand, for example, when a mode hop occurs at the wavelength λhp, the wavelength dependency of the power P changes discontinuously at the wavelength λhp as shown in FIG. The abrupt change | ΔP | or the absolute value | dP / dt | of the derivative with respect to the time changes sharply and to a large value. However, in any example, the temperature of the light source is constant.

  Therefore, the presence or absence of a mode hop can be determined by calculating the abrupt change | ΔP | of the output signal itself of the light receiver 60 or the absolute value | dP / dt |

  The mode hop detection means 61 performs the above calculation on the output signal of the light receiver 60, and compares the obtained | ΔP | or | dP / dt | with the respective threshold values set in advance. The presence / absence of a hop and its occurrence timing are detected. This generation timing is measured with reference to the rise and fall of the drive signal for sweeping.

  Here, the principle of mode hop generation will be briefly described. When the mode hop does not occur, as shown in FIG. 7A, the external frequency is changed with respect to the wavelength change of one specific longitudinal mode (solid line: Fabry-Perot mode) among the longitudinal modes that can be oscillated by the semiconductor laser 22. The specific longitudinal mode continues to be selected as the center wavelength of the selection characteristic F by the resonator changes.

  On the other hand, when a mode hop occurs, the center wavelength of the selection characteristic F by the longitudinal mode and the external resonator is shifted, and the selection characteristic F is selected at a certain wavelength λhp as shown in FIG. The vertical mode is jumped from the desired mode (solid line) to the next mode (dashed line).

  Here, when the injection current I ′ or the reverse bias voltage Vr to the phase adjustment region 130 of the semiconductor laser 22 is changed, the oscillation wavelength of the longitudinal mode can be continuously changed, and the center of the selection characteristic F of the resonator. By changing the oscillation wavelength of the longitudinal mode so as not to deviate significantly from the wavelength, jumping to the adjacent longitudinal mode, that is, mode hopping can be suppressed.

  The phase adjustment region 230 cannot be controlled unless the mode hops are generated under the same wavelength with high reproducibility. However, in the present invention, the rotating mirror 30 operates stably as described above. Therefore, by adjusting or variably controlling the injection current and the reverse voltage bias with respect to the phase adjustment region 230 of the semiconductor laser 22, there is no jump in the output optical power as shown in FIG. Wavelength sweep can be easily realized.

  Here, as a control to the phase adjustment region 230 of the semiconductor laser 22, when a mode hop is detected in the previous wavelength sweep, the injection current I ′ and the reverse voltage bias Vr are changed by a predetermined amount to change to the next wavelength. When the mode hop is detected by the wavelength sweep, the semi-fixed control mode for finding the constant injection current I ′ and the reverse voltage bias Vr in which the mode hop is not generated and the constant injection current I ′ and the reverse voltage bias Vr are repeated. A dynamic control in which the injection current I ′ and the reverse voltage bias Vr at the mode hop generation timing are changed so as to suppress the mode hop can be considered.

  In the case of dynamic control, the change pattern of the injection current I ′ and the reverse voltage bias Vr is arbitrary. For example, as shown by the injection current (or the reverse bias voltage) a in FIG. A pattern that changes step by step at a slightly earlier timing, or a pattern that changes continuously with a straight line (or a curved line) having an inclination, such as an injection current (or a reverse bias voltage) b, may be used.

  Further, as one control example, when a process of finding a constant injection current I ′ and a reverse voltage bias Vr at which no mode hop occurs in the semi-fixed control mode is first performed, and all the mode hops are lost in the process. Finishes the process, and when the mode hops are not completely eliminated by this process, the injection current I ′ and the reverse voltage bias Vr that minimize the generation of the mode hops are set and then injected every mode hop by the dynamic control mode. The generation of the current I ′ and the reverse voltage bias Vr is suppressed by changing the current I ′ and the reverse voltage bias Vr. By such control, mode hops can be easily eliminated.

  It should be noted that means for notifying the outside by letters, lamps or sounds may be provided during the period when the mode hop is occurring.

  The reason why the mode hop can be suppressed only by using such a simple control method is that the wavelength sweeping method using the rotating mirror 30 having the MEMS structure is very stable. Even if a mode hop occurs, the reproducibility is very good. Therefore, it is easy to suppress this by controlling the injection current I ′ and the reverse voltage bias Vr, and easy high-speed wavelength sweep without a mode hop. realizable.

  In the above embodiment, the reflecting plate 32 is formed symmetrically with respect to the connecting portions 33 and 34, and one end side thereof is used as a light reflecting portion, and an external force is received at the other end side. The form of external force application is not limited to the above embodiment.

  For example, like the reflecting plate 32 of the rotating mirror 30 shown in FIG. 9, the lateral length on the other end side for receiving an external force is made shorter than the one end side, and the width (length in the vertical direction) is widened. Thus, the left and right rotational moments may be balanced. In this case, the horizontal width of the entire rotating mirror 30 can be reduced.

  Moreover, although the said embodiment demonstrated the case where external force was periodically given to the one end side of the reflecting plate 32, and the reciprocating rotation was carried out, the position which provides external force is arbitrary, for example, as shown in FIG. By arranging the electrode plates 35 and 36 at both ends on the back side of the plate 32 and applying the signals V1 and V2, respectively, the reflection plate 32 can be reciprocally rotated as described above.

  The shape of the electrode plates 35 and 36 is also arbitrary, and a comb-like shape may be used in addition to the flat shape as in the above embodiment.

  In addition to the electrostatic force described above as an external force, an external force can be applied electromagnetically by a combination of an electromagnet (or coil) and a magnetic material, a combination of a permanent magnet and an electromagnet (or coil), etc. Furthermore, the ultrasonic vibrator having a frequency equal to the natural frequency of the reflecting plate 32 can be applied to the base 21 and the reflecting plate 32 can be rotated back and forth at the natural frequency. In addition, when giving external force other than electrostatic in this way, the material of the rotation mirror 30 does not need to have electroconductivity.

  In the above embodiment, the reflecting surface 24 a of the fixed mirror 24 is parallel to the diffraction groove 25 b of the diffraction grating 25, and the optical path from the semiconductor laser 22 to the reflection plate 32 through the collimator lens 23, the fixed mirror 24, and the diffraction grating 25. However, this does not limit the present invention, and the semiconductor laser 22 and the collimating lens 23 are separated by a plane obtained by extending the reflecting surface 32a of the reflecting plate 32. Any one of the two spaces including the diffraction grating 25 can be arranged at any position, and the direction of the reflecting surface 24a of the fixed mirror 24 may be set in accordance with the position.

  For example, as shown in FIG. 11, the semiconductor laser 22 and the collimating lens 23 are arranged so as to be aligned vertically with respect to the base 21 so that the optical axis thereof is parallel to the diffraction groove 25 a of the diffraction grating 25. May be received by the fixed mirror 24 of the reflecting surface 24 a that forms an angle of 45 degrees with respect to the upper surface of the base 21 and may enter the diffraction surface 25 a of the diffraction grating 25. The semiconductor laser 22, the collimating lens 23, and the fixed mirror 24 are supported by the support member 41.

  Moreover, in the said embodiment, although it has shown with the structure which stood the diffraction grating 25 and the rotation mirror 30 on the base 21, so that arrangement | positioning of each part can be understood easily, the semiconductor laser 22, the collimating lens 23, fixed The support form of each part including the mirror 24 is arbitrary.

  For example, as shown in FIG. 12, the frame 31 of the rotating mirror 30 is supported by support members 51 and 52 erected on both upper ends of the flat base 50, and the support member 53 erected on the upper part of the base 50. A structure in which the diffraction grating 25 is supported, and the semiconductor laser 22, the collimating lens 23, and the fixed mirror 24 are supported by the support member 54 erected in the vicinity of the support member 53, and the light receiver 60 is supported by the support member 55. Good. The support members 53 to 55 may be integrated.

  Further, as shown by the dotted line in FIG. 12, the semiconductor laser 22, the collimating lens 23, the fixed mirror 24, and the light receiver 60 are also supported by the support members 53 to 55 on the other end side of the reflecting plate 32, so It is also possible to configure so that two systems can be emitted. In this case, if the two wavelength variable ranges are the same, a two-channel wavelength swept light source can be realized, and if the two wavelength variable ranges are changed, a broader wavelength swept light source can be realized.

  Although not shown, one or two sets of the semiconductor laser 22, the collimator lens 23, the fixed mirror 24, and the light receiver 60 are arranged on the opposite side of the reflector 32, and the number of outgoing light systems is further increased. It is also possible to construct a wavelength-swept light source having no multi-channel and broadband mode hops.

  The above-described wavelength swept light source has a multi-channel structure or a broadband structure in which the optical path does not intersect the rotating mirror 30, and the semiconductor laser 22 as well as the diffraction grating 25 is provided on one surface of the reflecting surface of the reflecting plate 32. This is an effect brought about by arranging the collimating lens 23, the fixed mirror 24 and the light receiver 60 together.

  In the above embodiment, the phase control range in which mode hops can be suppressed depends on the element characteristics of the semiconductor laser 22, but in the case of the semiconductor laser 22 having the general structure described above, the waveguide layer and the p-type cladding in the phase adjustment region 230. Since the conduction band discontinuity ΔEc with the layer is small, an overflow of electrons is likely to occur, and the maximum phase adjustment amount is limited by this overflow, and the phase control range in which mode hops can be suppressed is also limited by this limitation.

  Therefore, if a semiconductor laser 22 having characteristics capable of adjusting the phase in a wider range is used, the control range for suppressing mode hops is expanded, which is further preferable.

Hereinafter, the semiconductor laser 22 in consideration of this point will be described.
FIG. 13 is an external view of the semiconductor laser 22 in which the phase adjustment amount by the phase adjustment region is greatly expanded, FIG. 14 is a plan view, and FIG. 15 is a cross-sectional view taken along line AA of FIG.

As shown in these drawings, the semiconductor laser 22 has an n-type semiconductor substrate 221 (hereinafter simply referred to as a substrate 221) made of InP (indium phosphorus). The substrate 221 also serves as an n-type cladding layer having a mesa structure, which will be described later, and has a substantially constant doping concentration (for example, 1 × 10 ¹⁸ / cm ³ ).

  A light emitting region 222 is formed on one end side (right end side in FIGS. 2 and 3) on the substrate 221, and a phase adjustment region 230 is formed on the other end side (left end side in FIGS. 14 and 15).

  In the light emitting region 222, an active layer 223 made of InGaAsP (indium, gallium, arsenic, phosphorus) with a substantially constant width (for example, 5 μm) is formed in the center on the substrate 221, and a first p-type cladding made of InP is formed thereon. The layer 224 is formed with a predetermined thickness (for example, 250 nm). The active layer 223 referred to here includes a multiple quantum well layer (MQW) and an SCH layer sandwiching it. Further, the distribution of the doping concentration of the first p-type cladding layer 224 in the thickness direction may be constant. However, as will be described later, the distribution is maximized within a certain range to reduce carrier leakage at the time of supplying current. The structure to prevent may be sufficient.

  On the other hand, a waveguide layer 231 made of InGaAsP is formed at the center on the substrate 221 on the phase adjustment region 230 side so as to be optically coupled to the active layer 223. Here, the active layer 223 has a gain with respect to light in a certain wavelength range, and is formed so as to guide light in the length direction. On the other hand, the waveguide layer 231 mainly has a waveguide function for the light, but may have a gain for the light.

  The energy gap of the waveguide layer 231 is set to be larger than the energy of the light, and the light is not absorbed by the waveguide layer.

  A second p-type cladding layer 232 is formed on the waveguide layer 231 so as to be continuous with substantially the same thickness as the first p-type cladding layer 224. The connection position between the active layer 223 and the waveguide layer 231 and the connection position between the first p-type cladding layer 224 and the second p-type cladding layer 232 coincide with each other.

  A third p-type cladding layer 240 made of InP is formed on and on the first p-type cladding layer 224 and the second p-type cladding layer 232.

  A buried layer 242 made of p-type InP is formed on the substrate 221 on the side of the active layer 223, and a buried layer 243 made of n-type InP is formed on the buried layer 242. .

  The boundary between the light emitting region 222 and the phase adjustment region 230 of the third p-type cladding layer 240 is a bridge portion 250 having a waveguide layer 231 optically coupled to the active layer 223 inside, and its left and right sides. It is comprised from the isolation | separation groove | channels 251 and 252 provided in the area | region. The bridge portion 250 has, for example, an element size described later, a length of 50 μm, a width of 15 μm, and a depth of about 7.5 μm.

  The isolation grooves 251 and 252 are formed not only by the third p-type cladding layer 240 but also by performing an etching process deeper than the interface between the buried layers 242 and 243 and the substrate 221.

  The longer length of the bridge portion 250 is advantageous in order to obtain a high separation resistance. However, the emission intensity decreases when the active region length decreases, or the maximum phase adjustment amount decreases when the phase adjustment region length decreases. Therefore, under the element length described later, the range is 25 μm to 100 μm.

  Further, the narrower the width of the bridge portion 250, the higher the separation resistance is obtained. However, it is necessary to provide a margin with respect to the width of the active layer 223 and the waveguide layer 231 (for example, 5 μm in the element size described later). In consideration of the size of the spot size, the range is 10 μm to 20 μm. The depth is set in the range of 5 μm to 10 μm because the hole leakage can be limited by etching deeper than the interface between the substrate 221 and the buried layer.

A contact layer 261 made of InGaAs is formed on the third p-type cladding layer 240 on the light emitting region 222 side with the separation grooves 251 and 252 as a boundary, and on the third p-type cladding layer 240 on the phase adjustment region 230 side. Similarly, a contact layer 262 made of InGaAs is formed, and a first upper electrode 271 made of gold (Au), platinum (Pt), and titanium (Ti) is formed on the contact layers 261 and 262, respectively. Two upper electrodes 272 are formed by vapor deposition. A lower electrode 273 made of gold (Au), germanium (Ge), or platinum (Pt) is formed on the entire lower surface of the substrate 221 by vapor deposition.
Note that an antireflection film (not shown) for preventing light reflection at the element end face 220a is formed on one element end face 220a of the semiconductor laser 22.

  Here, a current is passed in the forward direction between the second upper electrode 272 and the lower electrode 273 on the phase adjustment region 230 side, and the current value is changed in a range of, for example, 0 to several tens of mA, so that the waveguide layer The occurrence of mode hops can be prevented by changing the refractive index of H.231. In addition, a reverse bias voltage Vr is applied between the second upper electrode 272 and the lower electrode 273 instead of the forward current, and the voltage is changed within a predetermined range (for example, from 0 to several volts), whereby the waveguide layer The occurrence of mode hops can be prevented by changing the refractive index of H.231.

  However, as described above, in the semiconductor light emitting device having the phase adjustment region 230 as described above, since the conduction band discontinuity (ΔEc) between the waveguide layer and the cladding layer is small, carrier overflow is likely to occur, and phase adjustment is performed. There was a problem that the possible range was narrow.

  Therefore, the inventors conducted various experiments on the semiconductor laser 22 having the above structure, and as a result, the characteristic distribution in the doping concentration of the second p-type cladding layer 232 on the waveguide layer 231 in the phase adjustment region 230, that is, It has been found that the maximum phase adjustment range can be expanded without flowing useless current by suppressing the overflow in the phase adjustment region by giving a distribution having a maximum within a predetermined distance from the waveguide layer 231.

  This is shown in FIG. As a result of various experiments, the doping concentration in the thickness direction of the second p-type cladding layer 232 in contact with the waveguide layer 231 in the phase adjustment region 230 as in the characteristic F is within a predetermined range from the upper end of the waveguide layer 231, particularly 25 By forming so as to have a maximum value in the range of ˜150 nm, the effect of suppressing the overflow of injected carriers in the phase adjustment region 230 is enhanced, and it is not necessary to flow a useless current that does not contribute to the change in the refractive index. It was found that high efficiency of the entire device can be realized. The characteristic F ′ is an example of an actual doping concentration distribution.

  Thus, it has been found that setting the maximum position relatively close to the waveguide layer 231 is effective for blocking carriers (electrons) that diffuse as reactive currents. In particular, improvement in efficiency in the high implantation region was confirmed.

  FIG. 17 is a graph showing the effect. On the right side of the graph, the horizontal axis is the injection current Ipc (mA) to the phase adjustment region, the vertical axis is the wavelength λ, and attention is paid to one Fabry-Perot mode. This is a measurement of the change in the wavelength λ when the current Ipc is gradually injected into the phase adjustment region.

  The lower characteristic of FIG. 17 (characteristic plotted with black circles) is set so that the doping concentration of the second p-type cladding layer becomes a maximum value at a distance of 35 nm from the upper end of the waveguide layer. These characteristics (characteristics plotted with black squares) are those of a conventional element in which the doping concentration of the second p-type cladding layer is made uniform.

  As is apparent from FIG. 17, the shift amount in the Fabry-Perot mode is already saturated at about 5 mA in the case of the conventional device having a uniform doping concentration, whereas the maximum shift amount is 2π. In the element of the present embodiment in which the maximum value is given to the predetermined distance by the doping concentration distribution, the shift amount is not saturated even when Ipc is about 5 mA, and the maximum shift amount is improved to 3π. I understand.

  The characteristic Q in the left graph of FIG. 17 shows the tendency of the wavelength change when the reverse bias voltage Vr is changed. By increasing the reverse bias voltage Vr, the wavelength can be changed in the long wavelength direction. In addition, the wavelength change amount (phase change amount) can be further widened by selectively using both.

As shown in FIG. 16, by setting the maximum value of the doping concentration to about 7 × 10 ¹⁷ / cm ³ , Zn diffusing into the waveguide layer 231 can be minimized, and loss due to holes can be reduced. It was possible to obtain a high conversion efficiency while minimizing the overflow of injected electrons and minimizing the overflow of injected electrons.

  Here, a characteristic distribution is given to the doping concentration of the second p-type cladding layer 232 in the phase adjustment region 230 to improve the conversion efficiency. However, the first p-type cladding layer 224 on the light emitting region 222 side also has, for example, FIG. Like the characteristic G (actually, for example, the characteristic G ′), the luminous efficiency can be improved by giving a distribution that maximizes in the range of 50 nm to 250 nm from the active layer. High efficiency can be realized by using together with the doping concentration distribution of 230.

  However, since the doping concentration distribution of the second p-type cladding layer 232 in the phase adjustment region 230 does not depend on the concentration distribution of the light emitting region 222, the above-described effect is exhibited regardless of the structure of the light emitting region 222.

  Further, as described above, the bridging portion 250 in which the active layer 223 and the waveguide layer 231 are optically coupled is left in the third p-type cladding layer 240 at the boundary portion between the light emitting region 222 and the phase adjustment region 230. Since the separation grooves 251 and 52 are provided, the separation resistance between the regions can be increased, and the leakage current from the light emitting region 222 to the phase adjustment region 230 is significantly reduced, and the bridge portion 250 is guided. Light reflection can be extremely reduced.

  The doping profile of the third p-type cladding layer 240 is important for determining the series resistance and the waveguide loss.

  If the series resistance in the film thickness direction of the light emitting region is high, mode hops are likely to occur. This is a phenomenon that occurs because the refractive index changes due to Joule heat generated by current injection, resulting in a long wave. The current difference from one mode in Fabry-Perot mode to the next mode is called the mode hop current ΔIhop. The higher the mode hop current ΔIhop, the wider the range of stable operation in one mode. Therefore, it is preferable.

  That is, in order to increase the mode hop current ΔIhop, it is desirable to set the doping concentration high and lower the series resistance. On the other hand, if the doping concentration in the lower layer region (the starting point portion in the thickness direction) close to the active layer 223 and the waveguide layer 231 is set high, the light absorption loss near the active layer 223 and the waveguide layer 231 increases.

Therefore, while the doping profile of the third p-type cladding layer 240 is set low in the lower layer region (starting point portion in the thickness direction) close to the active layer and the waveguide layer, the series resistance of the entire third p-type cladding layer is reduced by the mode hop. It is necessary to adjust so as to be within a predetermined range that is unlikely to occur .

The characteristics of one doping profile are shown in FIG. In this characteristic H, the doping concentration is increased from the lower layer region (start point portion in the thickness direction) close to the active layer and the waveguide layer toward the active layer and the upper layer region (end point portion in the thickness direction) far from the waveguide layer. 3.0 × 10 ¹⁷ , 8.5 × 10 ¹⁷ , and 1.8 × 10 ¹⁸ (/ cm ³ ) are gradually increased. As a result, the series resistance value of 0.5Ω is obtained. Empirically speaking, the series resistance is preferably 0.7Ω or less. For this purpose, the doping concentration at the end point in the thickness direction is set to 1.0 × 10 ¹⁸ (/ cm ³ ) to 2.5 × 10 ¹⁸ ( / Cm ³ ) has been confirmed.

  Further, by limiting the layer thickness t to the range of 2 μm to 3.2 μm, the loss resistance in the vicinity of the waveguide layer 231 is reduced, and the separation resistance from the light emitting region 222 to the phase adjustment region 230 is set to 1 kΩ. I was able to. If the layer thickness t of the third p-type cladding layer 240 is made smaller, the separation resistance can be further increased, but it becomes smaller than the spot size of the emission end face, increasing the loss and making it impossible to guide the wave. Also, if the layer thickness is larger than 3.2 μm, the resistance of the third p-type cladding layer 240 decreases and the isolation resistance between the regions decreases, so the above layer thickness range is preferable.

  In FIG. 19, the doping concentration is increased in three steps, but the number of change steps may be two or four or more, or may be increased linearly as shown by the characteristic H ′ in FIG. A step change and a linear change may be used in combination.

Next, a method for manufacturing the semiconductor laser 22 will be described.
First, as shown in FIG. 21, a substrate 121 made of n-type InP with a doping concentration of 1 × 10 ¹⁸ (/ cm ³ ) is prepared, and then made of InGaAsP by MOVPE as shown in FIG. An active layer 122 is grown and further a first p-type cladding layer 123 is laminated thereon. The active layer 122 referred to here includes a multiple quantum well layer (MQW) and an SCH layer sandwiching it. Further, the thickness of the first p-type cladding layer 123 is, for example, 250 nm, and the distribution of the doping concentration in the thickness direction may be constant. However, as described above, the distribution is maximized within a certain range. It may be a structure that prevents carrier leakage at the time of current supply.

Next, as shown in FIG. 23, an insulating film 125 made of SiO ₂ or SiNx is deposited on the first p-type cladding layer 123 by a plasma CVD method or the like, and a resist 126 is applied thereon.

  Subsequently, as shown in FIG. 24, the resist in the phase adjustment region is removed by photolithography, and the insulating film in the region not covered with the resist 126 ′ is removed by etching as shown in FIG.

  Further, the remaining resist 126 'is removed as shown in FIG. 26, and the first n of the region not covered with the insulating film 125' as shown in FIG. 27 is removed by etching using the insulating film 125 'as a mask. The mold cladding layer 123 and the active layer 122 are removed. Here, the InP of the first n-type cladding layer 123 is etched using, for example, a phosphoric acid etchant of hydrochloric acid, and the active layer 122 using hydrochloric acid + hydrogen peroxide + water.

  Next, as shown in FIG. 28, a waveguide layer 131 made of InGaAsP is grown on the etched portion of the substrate 121 so as to be continuous with the active layer 122 ', and then made of InP. The second p-type cladding layer 132 is laminated so as to be substantially the same height as the first p-type cladding layer 123 ′.

  Next, after the remaining insulating film 125 'is peeled off, an insulating film 140 is newly deposited on the entire surface as shown in FIG. 29, and a resist 141 is applied thereon.

  Then, in order to manufacture the mesa structure, the central portion of the resist 141 is left by photolithography as shown in FIG. 30, and both sides thereof are removed.

  Further, using the resist 141 'remaining in a linear shape with a certain width as a mask, both sides of the insulating film 140 are removed by etching as shown in FIG.

  Subsequently, the remaining resist 141 'is stripped and removed, and etching is performed using the insulating film 140' as a mask. As shown in FIG. 32, the active layer 122 '(223) having a substantially continuous trapezoidal shape (mesa structure) is obtained. And a waveguide layer 131 ′ (231), and a trapezoidal continuous first p-type cladding layer 123 ′ (224) and second p-type cladding layer 132 ′ (232) are formed thereon.

  Next, as shown in FIG. 33, a buried layer 142 (242) made of p-type InP and a buried layer 143 (243) made of n-type InP are formed on both sides of the active layer 223 and the waveguide layer 231. Then, as shown in FIG. 34, a third p-type cladding layer 145 (240) made of InP is grown thereon, and a contact layer 146 made of InGaAs is further formed thereon.

  As shown in FIG. 35, the first upper electrode 171 made of Au, Ti, and Pt is deposited on the contact layer 146 where the light emitting region 222 is to be formed, and the phase adjustment region 230 is also formed. The second upper electrode 172 made of Au, Ti, Pt is vapor-deposited, and the lower surface side of the substrate 121 is polished to vapor-deposit a lower electrode 173 made of Au, Ge, Pt.

  Finally, the semiconductor laser 22 shown in FIG. 13 is completed by performing an etching process for forming the bridge portion 250 and the separation grooves 251 and 252 described above.

  Although this process is not described in detail, an etching process is performed by using a resist as a mask in a photolithography process. For the contact layer 146, for example, a third p-type cladding is formed using a sulfuric acid-based etchant (selective etchant). Etching is performed using the layer (InP) 145 (240) as an etch stop layer. The third p-type cladding layer 145, the buried layers 142 and 143, and the substrate 121 are etched using, for example, a hydrochloric acid phosphoric acid etchant. Thereby, an element having a length of 1000 μm, a width of 400 μm, and a thickness of 100 μm is completed.

  The basic structure of the semiconductor laser 22 ′ shown in FIG. 36 is the same as that of the semiconductor laser 22, but a mesa type including the active layer 223 and the first p-type cladding layer 224 is used to suppress the end surface reflection component on the light emitting region side. The front end side of the waveguide is formed so as to obliquely intersect with the element end face 220a in a non-orthogonal state (in other words, the element end face 220a and the optical axis of light guided through the active layer 223 intersect in a non-orthogonal state) Thus, the reflection component at the element end face is prevented from returning to the active layer 223.

  The waveguide layer 231 on the phase adjustment region side is formed so as to obliquely intersect the element end surface 220b on the phase adjustment region side in a non-orthogonal state, and the optical axis of the emitted light is obliquely intersected with the element end surface. Alternatively, the crossing angle between the active layer 223 and the element end face 220a and the crossing angle between the waveguide layer 231 and the element end face 220b may be non-orthogonal.

  20... Swept light source, 21... Base, 22... Semiconductor laser, 23... Collimating lens, 24... Fixed mirror, 25 .. diffraction grating, 30. …… Reflecting plate, 32a …… Reflecting surface, 33, 34 …… Connecting portion, 35, 36 …… Electrode plate, 37 …… Spacer, 40 …… Mirror drive device, 50 …… Base, 51 to 55 …… Support member, 60... Receiver, 61... Mode hop detection means, 65... Phase adjustment region control means, 70... Temperature sensor, 71. 122... Light emitting region, 133... Phase adjustment region, 221... Substrate, 222... Light emitting region, 223... Active layer, 224. Waveguide layer, 232 ... 2nd p-type ,... 3rd p-type cladding layer, 250... Bridge portion, 251, 252... Separation groove, 261, 262... Contact layer, 271. Upper electrode, 273 ... Lower electrode

Claims (9)

A base (21);
A semiconductor laser (22) fixed on the base and having at least one light emitting end face having a low reflectivity surface;
A collimator lens (23) fixed on the base and converting the light emitted from the low reflectance surface of the semiconductor laser into parallel light;
A predetermined diffraction surface (25b) in which grooves for diffracting light are formed in parallel, and light emitted from the collimating lens is orthogonal to the grooves and non-orthogonal to the diffraction surface A diffraction grating (25) fixed on the base in a state of being incident at a predetermined incident position at an incident angle;
The collimating lens has a reflecting surface facing the diffraction surface of the diffraction grating, and is rotatable in a plane that is parallel to the groove of the diffraction grating and that is orthogonal to the diffraction surface about an axis at a specific position. Of the diffracted light with respect to the light emitted from and incident on the diffraction surface of the diffraction grating, the light along the optical axis perpendicular to the reflection surface is reflected and returned to the diffraction grating by a reverse optical path, and the returned light is incident A rotating mirror (30) for returning to the semiconductor laser via the collimating lens with the same optical axis as the optical path;
A distance r from the rotation center of the rotation mirror to the predetermined incident position of the diffraction grating, a distance L2 from the rotation center to a plane obtained by extending the reflection surface, and the diffraction grating from the effective resonance end surface of the semiconductor laser Between the optical path length L1 to the predetermined incident position and the light incident angle α to the diffraction surface of the diffraction grating,
r = (L1-L2) / sin α
Establishing the relationship
In response to a change in the angle of the reflecting surface of the rotating mirror, the resonator length from the semiconductor laser to the reflecting surface of the rotating mirror via the collimating lens and the diffraction surface of the diffraction grating is changed. In the wavelength sweep light source of the Litman method external resonator type that continuously changes the wavelength of the emitted light within a predetermined range,
The semiconductor laser is
An n-type semiconductor substrate (221), an active layer (223) formed on the n-type semiconductor substrate, and a first p-type cladding layer (224) stacked on the active layer, and for current injection A light emitting region (222) that receives and emits light and guides the light along the active layer;
A waveguide layer (231) having an energy gap larger than the energy of the light on the n-type semiconductor substrate and optically coupled to the active layer of the light emitting region, and laminated on the waveguide layer The second p-type cladding layer (232) has a structure including a phase adjustment region (230) in which a refractive index with respect to light from the light emitting region changes according to a forward injection current or a reverse bias voltage,
The rotating mirror is
A frame (31) fixed to the base; a reflector (32) disposed on the inner side of the frame and having the reflecting surface formed on one side; and an outer edge of the reflector and an inner edge of the frame And a pair of connecting portions (33, 34) that are twistable and connect to each other and are aligned in a straight line parallel to the grooves of the diffraction grating, and periodically apply a force to the end of the reflecting plate. With the dynamic drive means (35, 36, 40), the reflector is reciprocally rotated around the connecting part to sweep the wavelength, and even when the mode hop is generated during the wavelength sweep, the mode hop is highly reproduced. It has a MEMS structure with rotational stability that can be generated at the same wavelength .
further,
Either a light fixed on the base and emitted from an end surface opposite to the one end surface of the semiconductor laser, or a zero-order diffracted light with respect to light incident on the diffraction surface of the diffraction grating from the semiconductor laser A light receiver (60) for receiving and outputting an electrical signal corresponding to the intensity of the received light;
While the output wavelength of the semiconductor laser is swept by the reciprocating rotation of the reflecting plate of the rotating mirror, the output of the light receiver is monitored, and a discontinuous change in the output signal of the light receiver caused by a mode hop is observed. Mode hop detection means (61) for detecting;
Phase adjustment region control means (65) for changing the forward injection current or the reverse bias voltage of the phase adjustment region of the semiconductor laser so as to eliminate the discontinuous change in the output signal of the light receiver detected by the mode hop detection means. ) To change the forward injection current or reverse bias voltage by a predetermined amount, repeat the process of sweeping the next wavelength and detecting the mode hop, so that the mode hop does not occur or the mode hop occurs the least A wavelength swept light source comprising phase adjustment region control means for finding a constant forward injection current or reverse bias voltage and performing semi-fixed control processing for holding the constant forward injection current or reverse bias voltage. .   The doping concentration in the thickness direction of the second p-type cladding layer in contact with the waveguide layer in the phase adjustment region of the semiconductor laser has a maximum value within a predetermined distance from the upper end of the waveguide layer. The wavelength swept light source according to claim 1.   3. The wavelength swept light source according to claim 2, wherein the predetermined distance range is 25 to 150 nm.   A third p-type cladding layer (240) is formed on the first p-type cladding layer in the light emitting region of the semiconductor laser and the second p-type cladding layer in the phase adjustment region, and the light emitting region of the third p-type cladding layer The wavelength-swept light source according to any one of claims 1 to 3, wherein a separation groove (251, 252) having a predetermined width is formed at a boundary portion with the phase adjustment region. The doping concentration at the end point in the thickness direction of the third p-type cladding layer is larger than the doping concentration at the start point in the thickness direction, and 1.0 × 10 ¹⁸ (/ cm ³ ) to 2.5 × 10 ¹⁸ (/ cm ^3). The wavelength swept light source according to claim 4, which is set in a range of   The semiconductor laser is formed such that a doping concentration in a thickness direction of the first p-type cladding layer in the light emitting region of the semiconductor laser has a maximum value within a range of 50 to 250 nm from an upper end of the active layer. Item 6. A wavelength-swept light source according to any one of Items 1 to 5.   The crossing angle between the element end face (220a) on the light emitting region side of the semiconductor laser and the optical axis of the light guided through the active layer, or the element end face (220b) on the phase adjustment region side and the waveguide layer The wavelength-swept light source according to claim 1, wherein at least one of the crossing angles with the optical axis of the guided light is non-orthogonal. When the mode hop remains in the semi-fixed control process, the phase adjustment area control means is synchronized with the reciprocating rotation of the reflector during the subsequent wavelength sweep, and the forward direction of the phase adjustment area of the semiconductor laser The wavelength sweep light source according to any one of claims 1 to 7, wherein dynamic control processing is performed to variably control the injection current or reverse bias voltage from the constant value for each remaining mode hop to suppress the generation thereof .   The semiconductor laser, the collimating lens, the fixed mirror, and the diffraction grating have a structure in which light emitted from the collimating lens is incident on the diffraction grating via a fixed mirror (24) fixed on the base. The optical axis extending from the semiconductor laser to the diffraction grating via the fixed mirror is arranged in a non-crossing state with the reflector of the pivot mirror so that it is disposed on one surface side of the reflector of the pivot mirror. The wavelength swept light source according to any one of claims 1 to 8, wherein

Images