JP6586028B2 - Semiconductor laser light source - Google Patents

Description

  The present invention relates to a semiconductor laser light source that operates in a single mode and is used in medium and long distance optical fiber communication.

  In recent years, with the increase in capacity of optical communication systems, digital coherent communication systems using multi-level phase amplitude modulation have begun to spread. Since this communication method is a method of transmitting a digital signal using light phase information, a laser light source having a stable optical oscillation frequency (wavelength), a small phase noise, and a narrow spectral line width is required as a light source. Semiconductor lasers are widely used as light sources for optical communications due to their small size and low cost, but distributed feedback (DFB) lasers that operate in a single mode are widely used, especially in medium and long distance optical communication systems. It has been.

  In addition, in a medium / long-distance optical communication system, it is common to use wavelength division multiplexing (WDM) technology to increase the transmission capacity per optical fiber so that any wavelength channel can be output to the light source. In addition, wavelength tunable characteristics are also required.

  FIG. 1 shows a schematic basic configuration of a wavelength tunable laser module mainly used at present. This laser module incorporates a wavelength locker 3 in order to receive a laser beam from a laser light source 1 composed of a semiconductor laser by being branched by a partially reflecting mirror 2 and to stabilize the light oscillation wavelength. The wavelength locker 3 is an optical component including an etalon filter serving as a wavelength reference, a monitoring photodiode (PD), and the like.

  The wavelength locker 3 uses the optical frequency dependence of the transmittance of the etalon filter, detects the optical oscillation frequency (wavelength) of the semiconductor laser light that has passed through the etalon filter, and detects the reference etalon By applying external electrical control to the semiconductor laser 1 so as to match the optical frequency of the filter, the optical oscillation frequency (wavelength) of the semiconductor laser is stabilized.

  In general, a semiconductor laser has a relatively large phase noise because its resonator size is small compared to other solid-state lasers and gas lasers. Since the external control as described above involves a certain time delay, even if the laser oscillation frequency is stabilized in the long term, fluctuations in the short-term oscillation frequency are large, and the optical spectrum of the oscillation laser beam is large. The line width of is quite wide. The line width of a typical semiconductor laser having a resonator size of several hundred μm is on the order of MHz.

  Currently, in the 100 Gbit / s digital coherent communication system that is widely used, the polarization multiplexing QPSK modulation method is used, and a light source having a line width of several hundred kHz is required. In this case, a variable wavelength distributed feedback (DFB) laser array, an external resonator type laser, or the like whose length is narrowed by increasing the resonator length to about 1 mm is used. In the future, a light source with a narrower spectral line width is expected to realize a large-capacity communication with a large multilevel.

M. Finot, et al., “Thermally tuned external cavity laser with micromachined silicon etalons: design, process and reliability,” Electronic Components and Technology Conference 2004 Proceedings, Vol. 1, pp. 818-823, 2004. K. Aoyama, et al., “Optical Negative Feedback for Linewidth Reduction of Semiconductor Lasers,” IEEE Photon. Technol. Lett., Vol. 27, No. 4, pp. 340-343, 2015.

  For example, as a configuration for narrowing the spectral line width to about 10 kHz, there is a so-called external resonator type laser in which an optical resonator for laser oscillation is formed outside a semiconductor chip having an optical amplification function. For example, a wavelength covering the entire C band of 1550 nm by an external resonator laser composed of a semiconductor amplifier, an external reflector, an etalon filter for selecting a wavelength, and a lens as shown in Non-Patent Document 1. Variable characteristics and line width characteristics of several tens of kHz are obtained.

  However, the external resonator type laser has a problem that it requires a large number of parts outside the semiconductor chip and needs to be assembled with high accuracy. In addition, in order to select one wavelength from a large number of resonance modes, at least two or more wavelength filters must be controlled, and the control circuit becomes complicated, and the inspection of wavelength characteristics is complicated. There is also a problem that there is.

  On the other hand, in a wavelength tunable laser based on a semiconductor laser such as a DFB laser configured by providing an optical resonator on a semiconductor chip, the temperature control of the semiconductor chip is used while maintaining the same oscillation mode in principle. Control is simple because the wavelength can be changed, but there is a limit to narrowing the line width. As mentioned above, the line width up to about 100kHz has been achieved by making the resonator long up to about 1mm, but if the resonator length is made longer than this, a uniform resonator will be formed due to manufacturing fluctuations in the semiconductor chip. There is a problem that becomes difficult.

  The present invention has been made in view of such problems, and by combining a semiconductor laser and a wavelength locker, a compact, good controllability, stable and narrow line width laser light source is realized.

  In order to achieve such an object, the present invention is characterized by having the following configuration.

(Structure 1 of the invention)
A light source block having a semiconductor laser chip having a semiconductor laser oscillating in a single mode, and a wavelength locker having an etalon filter having a reference optical frequency used for controlling the optical oscillation frequency of the semiconductor laser · in the semiconductor laser light source constituted by a block, a portion of the reflected light from the etalon filter of the wavelength locker-block is configured to return to the semiconductor laser chip in the light source block, a single-mode When the optical oscillation frequency of the semiconductor laser that oscillates increases, the intensity of reflected light returning from the etalon filter increases, and conversely, when the optical oscillation frequency decreases, the intensity of reflected light returning from the etalon filter decreases. that operating the etalon filter at the operating point as a filter characteristic such that The semiconductor laser light source for the symptoms.

(Configuration 2)
In the semiconductor laser light source according to the first aspect of the invention, the semiconductor laser that oscillates in a single mode is configured by either a distributed feedback laser having a wavelength selection function by a diffraction grating or a distributed reflection laser. A semiconductor laser light source.

(Structure 3 of the invention)
3. The semiconductor laser light source according to claim 1, wherein the semiconductor laser chip having a semiconductor laser that oscillates in a single mode comprises N (≧ 1) semiconductor lasers, N A semiconductor laser light source comprising a tunable laser in which a one-to-one optical multiplexer and a semiconductor amplifier are integrated.

(Structure 4 of the invention)
In the semiconductor laser light source according to any one of configurations 1 to 3 ,
An optical isolator is provided on the optical path ahead of the light output side of the wavelength locker block,
A semiconductor laser light source, wherein an optical isolator is not provided on an optical path in which a part of reflected light from the etalon filter in the wavelength locker block returns to the semiconductor laser chip on the light source block.

(Structure 5 of the invention)
In the semiconductor laser light source according to any one of configurations 1 to 4 ,
Two functional blocks of the light source block and the wavelength locker block are arranged in a package,
Each functional block is mounted on a TEC (Thermo-electric cooler) element and is configured to be able to adjust the temperature independently,
The wavelength locker block includes
A first partially reflecting mirror for branching laser light directly incident from the light source block;
A second partial reflection mirror for further branching one of the branched lights from the first partial reflection mirror;
The etalon filter that receives one of the branched lights from the second partially reflecting mirror;
An intensity monitoring photodiode for receiving the other of the branched lights from the second partial reflection mirror;
A wavelength monitoring photodiode that receives the transmitted light from the etalon filter;
A semiconductor laser light source, wherein a temperature monitoring thermistor is mounted.

(Structure 6 of the invention)
In the semiconductor laser light source according to any one of configurations 1 to 5 of the invention,
2. A semiconductor laser light source, wherein two partial reflection surfaces constituting the etalon filter are arranged perpendicular to a light axis.

(Structure 7 of the invention)
In the semiconductor laser light source according to Structure 5 of the invention,
A semiconductor laser light source characterized by controlling the optical oscillation frequency of the semiconductor laser based on at least an output current of the wavelength monitoring photodiode.

  As described above, according to the present invention, the reflected light from the etalon filter in the wavelength locker block is positively returned to the semiconductor laser itself, thereby enabling a narrow line width operation while stabilizing the wavelength.

It is a figure which shows the basic composition of the outline of the conventional wavelength tunable laser module. It is a top view of the package of the semiconductor laser light source in the Example of this invention. It is a top view of the package of the conventional semiconductor laser light source. It is a figure which shows the optical frequency characteristic of the monitor output current ratio (PD1 / PD2) in the Example of this invention.

  The operation principle of the present invention will be described below. First, the negative feedback action of the etalon filter on the laser light oscillation frequency will be described.

  An etalon is an optical element having two partially reflecting surfaces facing each other at a predetermined interval, and a wavelength filter can be configured using multiple interference of reflected light and transmitted light. The etalon filter has a filter characteristic in which the transmitted light intensity and the reflected light intensity change periodically according to the optical frequency interval determined by the interval between the partial reflection surfaces and the wavelength of light.

  When an etalon filter is provided on the emission side of the semiconductor laser and the return light (reflected light) from the etalon filter is arranged so that it can be fed back to the semiconductor laser, the optical oscillation frequency of the semiconductor laser changes in the increasing direction. think of.

  If the operating point on the filter characteristics is set so that the intensity of the reflected light returning from the etalon filter increases when the optical oscillation frequency of the semiconductor laser increases, the intensity of the reflected light returning to the semiconductor laser is Increased and amplified by stimulated emission in the semiconductor gain medium. As a result, carriers (electrons and holes) in the semiconductor gain medium of the semiconductor laser are consumed, and the carrier density decreases.

  Then, due to the carrier-plasma effect, the refractive index of the semiconductor gain medium changes in the increasing direction, and the optical effective length of the optical resonator of the semiconductor laser becomes longer. As a result, the oscillation wavelength increases and the optical oscillation frequency decreases. Change direction. On the contrary, when the light oscillation frequency of the semiconductor laser is changed in a decreasing direction, a change in the opposite direction is generated and the light oscillation frequency is increased.

  By setting the operating point of the etalon filter in this way, a direct and rapid light negative feedback action to the optical oscillation frequency of the semiconductor laser works via the return light (reflected light) from the etalon filter, The spectral line width can be narrowed. For example, Non-Patent Document 2 reports an example of narrowing the line width using optical feedback by an etalon filter.

  The present invention uses the etalon filter mounted on the wavelength locker to stabilize the wavelength by controlling the external laser light source, which is the original use of the wavelength locker, as well as to the laser light source described above directly and quickly. It is also possible to realize narrow line width operation by negative feedback operation of light. Thus, with the configuration of the present invention, it is possible to realize a small-width light source that is small, stable, and has good controllability.

Embodiments of the present invention will be described below with reference to the drawings.
(Embodiment)
FIG. 2 is a plan view of a semiconductor laser light source package according to an embodiment of the present invention.

  In the semiconductor laser light source according to the present embodiment, two functional blocks of the light source block 102 and the wavelength locker block 103 are arranged in the package 101. Although not shown in the drawings, each functional block is mounted on a TEC (Thermo-electric cooler) element, and the temperature can be adjusted independently.

  The configuration of the light source block 102 is a semiconductor laser chip 104 in which a semiconductor laser that oscillates in a single mode and has a wavelength variable function is mounted on a metal carrier such as CuW that is an alloy of copper (Cu) and tungsten (W). A lens 106 for converting emitted light from the semiconductor laser into a parallel beam and a thermistor 105 for monitoring temperature are mounted. In the figure, the locus of the laser beam is indicated by a dotted arrow.

  The configuration of the wavelength locker block 103 includes a first partially reflecting mirror 110 for partially reflecting a part of the laser light directly incident from the light source block 102 onto the carrier, and further using the partially reflected light for wavelength monitoring. And a second partial reflection mirror 111 for branching to intensity monitor, an etalon filter 112 for receiving reflected light for wavelength monitoring, an intensity monitor photodiode 113 for receiving transmitted light for intensity monitoring, and an etalon filter 112 A wavelength monitoring photodiode 114 and a temperature monitoring thermistor 115 that receive the transmitted light are mounted.

  The light transmitted through the wavelength locker block 103 is transmitted through the transparent window 116 and emitted from the package 101, and then collected by the lens 117 and optically coupled to the optical fiber 120. In order to remove unnecessary return light from the outside from the optical fiber 120, an optical isolator 118 is provided on the end face of the fiber ferrule 119. The package 101 is hermetically sealed in order to improve the reliability of the semiconductor laser.

  Although not shown in the drawings, all components such as a semiconductor laser, a photodiode, and a thermistor are electrically wired and connected to terminals provided outside the package. The laser light source can be operated. The signal line of the electrical control signal from the wavelength locker block to the light source block for the conventional wavelength stabilization control operation is the same, though not shown.

  In addition, the laser beam branched by the two partial reflection mirrors is expressed as reflected light and transmitted light, but there are advantages and disadvantages such as restrictions on the arrangement of each component on the wavelength locker block. Since it is clear that the usage of the transmitted light may be reversed, the reflected light and the transmitted light can be said to be one of the branched lights and the other.

  In this embodiment, a distributed feedback (DFB) laser array is used as the semiconductor laser. A plurality of (N (≧ 1), N = 4 in FIG. 2) distributed feedback lasers 107 are arranged in a semiconductor laser chip 104 to form a laser array, and one N-to-1 optical multiplexer 108 is used. A semiconductor amplifier 109 for optical amplification after being integrated into a single optical waveguide is integrated. By selecting one of the four distributed feedback lasers 107 having different oscillation wavelengths constituting the laser array to oscillate, and changing the temperature of the laser chip while observing the output of the thermistor 105, the oscillation wavelength Is a wavelength tunable laser capable of changing the wavelength in a wide range.

  In this embodiment, a distributed feedback (DFB) laser 107 is used as a semiconductor laser. However, a distribution in which a diffraction grating for selecting a wavelength is provided in a portion different from the semiconductor gain region on the chip. A reflective (DBR) laser may also be used.

(Operation example of the present invention)
Hereinafter, an actual operation example of the present invention will be described with reference to FIG.
Laser light output from the semiconductor laser chip 104 on the light source block 102 having a semiconductor laser oscillating in a single mode is converted into a parallel beam by the lens 106 and proceeds to the wavelength locker block 103.

  The laser light that has passed through the first partial reflection mirror 110 of the wavelength locker block 103 is emitted from the transparent window 116, while part of the reflected laser light is reflected by the second partial reflection mirror 111 by the intensity monitoring photo. The light is guided to the diode 113 and the wavelength monitoring photodiode 114. Each light intensity is observed by being converted into a current value of a photodiode, and wavelength stabilization control is performed by an electric signal from the wavelength locker block to the light source block as in the conventional case.

  An etalon filter 112 is inserted in the optical path to the wavelength monitoring photodiode 114. Since the transmittance of the etalon filter changes according to the wavelength of light (optical frequency), the current value of the wavelength monitoring photodiode 114 also changes according to the wavelength (optical oscillation frequency).

  Part of the return light (reflected light) from the etalon filter 112 follows a path opposite to that of the etalon filter 112 and reaches the semiconductor laser on the semiconductor laser chip 104 in the light source block 102, and negative feedback action due to the light described above. Acts to narrow the spectral line width.

(Comparison with conventional semiconductor laser light source)
For comparison, a plan view of a conventional semiconductor laser light source package is shown in FIG. In the configuration of the conventional example of FIG. 3, the description of the same part as the semiconductor laser light source of the present invention of FIG. 2 is omitted, but an optical isolator 218 is always inserted in the optical path between the light source block and the wavelength locker block. . This is conventionally required for the reason that unnecessary reflected light from the wavelength locker block returns to the laser light source side to destabilize the laser oscillation operation.

  For this reason, in the conventional configuration of FIG. 3, although the light oscillation frequency is stabilized by external control of the laser light source, the negative feedback action by the direct light from the etalon filter to the semiconductor laser does not work. The narrowing of the spectral line width has not been achieved.

  In the conventional example of FIG. 3, the optical isolator 218 is mounted on the light entrance portion of the wavelength locker block, but there is also an example in which the optical isolator is mounted on the exit side of the light source block. In any case, conventionally, in order to prevent unnecessary reflected light from the wavelength locker block from returning to the laser light source side, in the optical path between the semiconductor laser of the light source block and the etalon filter of the wavelength locker block In addition, an optical isolator is inserted, and the negative feedback action as in the present invention does not work.

  In the present invention, no optical isolator is intentionally provided in the optical path between the light source block and the wavelength locker block, and the operating point of the filter characteristic of the reflected light of the etalon filter is set to a point where the negative feedback action works. The reflected light from the etalon filter is actively used on the semiconductor laser light source side to narrow the spectral line width.

(Optical frequency characteristics of the present invention)
FIG. 4 shows the light of the value obtained by dividing the output current (PD1) of the wavelength monitoring photodiode 114 by the output current (PD2) of the intensity monitoring photodiode 113 when the optical frequency is actually changed in this embodiment. Shows frequency characteristics. This characteristic is determined by the output current (PD1) of the wavelength monitor photodiode 114 that detects light transmitted through the etalon filter, and the output current (PD2) of the intensity monitor photodiode that detects light on the etalon filter input side. Since it is standardized, it is equivalent to the optical frequency characteristic (filter characteristic) of the light transmittance of the etalon filter.

  In this embodiment, by adjusting the temperature of the wavelength locker block to an appropriate value, the operating point at which the slope of the transmission characteristic of the etalon filter becomes a negative maximum value at 193100 GHz in FIG.

  In FIG. 4, the value of the monitor current ratio (PD1 / PD2) at this time is 1.26. Therefore, by electrically controlling the optical oscillation frequency of the semiconductor laser from the outside so as to keep the monitor current ratio (PD1 / PD2) at 1.26, it can be accurately kept at 193100 GHz and the wavelength is stabilized. Can be controlled.

  Note that the transmission characteristics of the etalon filter periodically change, and the period depends on the optical distance between the two mirrors of the etalon filter. In this embodiment, an etalon filter designed and manufactured so that the period is 50 GHz is used. Therefore, the laser light oscillation frequency (wavelength) can be stabilized at the optical frequency arranged at intervals of 50 GHz.

  In addition, since the transmitted light frequency of the etalon filter can be changed depending on the temperature, it can be stabilized to an arbitrary wavelength by controlling the temperature.

  In this embodiment, the two partial reflection surfaces of the etalon filter are arranged perpendicular to the light axis. Therefore, a part of the reflected light from the etalon filter can return to the semiconductor laser chip on the laser light source block along the original optical path. In the conventional example (FIG. 3), since the optical isolator 218 is arranged on the optical path returning to the light source block side, the reflected light from the etalon filter never returns to the semiconductor laser. It is possible to return to the laser as it is. In this embodiment, the operating point of the etalon filter is set as follows. As a result, the above-described negative feedback action of the optical frequency works, and the action of narrowing the spectral line width works.

  In FIG. 4, when the transmitted light frequency is 193100 GHz, it is an operating point where the transmittance of the etalon filter has a negative slope with respect to the optical frequency. On the other hand, it has a positive slope. For example, when the optical oscillation frequency is increased, the reflected light intensity is increased.

  Therefore, as described above, a negative feedback action to the optical oscillation frequency of the semiconductor laser works, and a narrow line width operation is achieved. Actually, the line width of the semiconductor laser light source with the configuration of the conventional example was about 500 kHz, but it could be reduced to about 10 kHz by applying the present invention.

  As described above, by applying the present invention, it is possible to obtain a small wavelength tunable laser light source capable of achieving both wavelength stabilization and narrow line width operation with a configuration substantially the same as that of the prior art.

  The semiconductor laser light source of the present invention can be generally used for an optical communication system. In particular, it is possible to provide a semiconductor laser light source that can be used in a transmitter / receiver of an optical communication system and is capable of stable and narrow line width operation.

101 Package 1, 102 Light source block 3, 103 Wavelength locker block 104 Semiconductor laser chip 105 Thermistor 106 Lens 107 Distributed reflection type (DFB) laser 108 Optical multiplexer 109 Semiconductor amplifier 2, 110, 111 Partial reflection mirror 112 Etalon filter 113 Intensity monitoring photodiode 114 Wavelength monitoring photodiode 115 Thermistor 116 Transparent window 117 Lens 118, 218 Optical isolator 119 Fiber ferrule 120 Optical fiber

Claims (7)

A light source block having a semiconductor laser chip having a semiconductor laser oscillating in a single mode, and a wavelength locker having an etalon filter having a reference optical frequency used for controlling the optical oscillation frequency of the semiconductor laser In a semiconductor laser light source composed of blocks,
Is configured such that a portion of the reflected light from the etalon filter of the wavelength locker-block returns to the semiconductor laser chip in the light source block,
When the optical oscillation frequency of the semiconductor laser that oscillates in a single mode increases, the intensity of the reflected light returning from the etalon filter becomes stronger,
Conversely, the semiconductor laser is characterized in that the etalon filter is operated at an operating point where the filter characteristic is such that the intensity of reflected light returning from the etalon filter becomes weak when the optical oscillation frequency decreases. light source. The semiconductor laser light source according to claim 1,
A semiconductor laser light source characterized in that the semiconductor laser oscillating in a single mode is constituted by either a distributed feedback laser having a wavelength selection function by a diffraction grating or a distributed reflection laser. The semiconductor laser light source according to claim 1 or 2,
The semiconductor laser chip having a semiconductor laser that oscillates in a single mode becomes a wavelength tunable laser in which a laser array including N (≧ 1) semiconductor lasers, an N-to-1 optical multiplexer, and a semiconductor amplifier are integrated. A semiconductor laser light source. In the semiconductor laser light source according to any one of claims 1 to 3 ,
An optical isolator is provided on the optical path ahead of the light output side of the wavelength locker block,
A semiconductor laser light source, wherein an optical isolator is not provided on an optical path in which a part of reflected light from the etalon filter in the wavelength locker block returns to the semiconductor laser chip on the light source block. The semiconductor laser light source according to any one of claims 1 to 4 ,
Two functional blocks of the light source block and the wavelength locker block are arranged in a package,
Each functional block is mounted on a TEC (Thermo-electric cooler) element and is configured to be able to adjust the temperature independently,
The wavelength locker block includes
A first partially reflecting mirror for branching laser light directly incident from the light source block;
A second partial reflection mirror for further branching one of the branched lights from the first partial reflection mirror;
The etalon filter that receives one of the branched lights from the second partially reflecting mirror;
An intensity monitoring photodiode for receiving the other of the branched lights from the second partial reflection mirror;
A wavelength monitoring photodiode that receives the transmitted light from the etalon filter;
A semiconductor laser light source, wherein a temperature monitoring thermistor is mounted. The semiconductor laser light source according to any one of claims 1 to 5 ,
2. A semiconductor laser light source, wherein two partial reflection surfaces constituting the etalon filter are arranged perpendicular to a light axis. The semiconductor laser light source according to claim 5 ,
A semiconductor laser light source characterized by controlling the optical oscillation frequency of the semiconductor laser based on at least an output current of the wavelength monitoring photodiode.

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