JP2009087956A - External resonator type variable wavelength laser and semiconductor optical amplifier built into the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve an external resonator type variable wavelength laser using a wavelength bandpass filter having residual transmission or reflection to some extent, or a variable wavelength reflection structure. <P>SOLUTION: The external resonator type variable wavelength laser incorporates a semiconductor gain region having a product ΓL of an optical confinement constant Γ and semiconductor gain region length L where the absolute value ΔLoss (dB) between loss at a wavelength λ1 in the reflection structure and loss at a wavelength λ2 becomes larger than a difference Δg (dB) between a gain at the wavelength λ1 in the semiconductor gain region and a gain at a wavelength λ2 in laser oscillation, when the reflection spectrum peak wavelength of the reflection structure is the arbitrary wavelength λ1 included in the variable wavelength range of the variable wavelength laser and is the wavelength λ2 indicating the maximum gain in the semiconductor gain region in laser oscillation. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an external resonator type wavelength tunable laser having a surface type reflection structure capable of varying a reflection spectrum peak wavelength, and a semiconductor optical amplifier built in the wavelength tunable laser as a semiconductor gain region.

  In recent years, with the rapid spread of the Internet, there has been a demand for further increase in communication traffic capacity. In order to cope with this, in the optical communication system, the transmission rate per single channel is improved, and the number of channels is increased by using a wavelength division multiplexing (WDM) system.

  WDM is a method of simultaneously transmitting a plurality of optical signals of different carrier wavelengths (channels), and can be transmitted by one optical fiber. Therefore, the communication capacity can be expanded according to the number of multiplexed channels. . For example, if modulation is performed at 10 gigabits / second per channel and 100 channels are transmitted through one common optical fiber, the communication capacity reaches 1 terabit / second.

  In recent medium and long-distance optical communications, the C band (1530 to 1570 nanometers) that can be amplified by an optical fiber amplifier (EDFA, erbium-doped fiber amplifier) is widely used. In addition, the L band (1570 to 1610 nanometers) may be used for further expansion of communication capacity.

  Generally, a WDM system requires a different laser device for each wavelength. For this reason, manufacturers and users of the WDM system need to prepare laser devices corresponding to the wavelengths of the standard channels. For example, if there are 100 channels, 100 types of laser devices are required, which increases inventory management and inventory costs.

  In view of this, there has been a demand for practical use of a wavelength tunable laser that covers the entire wavelength of the C band (or L band) with a single laser in a WDM system, particularly in a medium to long distance communication system. If the entire C band (or L band) can be covered with a single laser device, the manufacturer and the user need only prepare a single laser device, greatly reducing inventory management and inventory costs. Can do.

  In order to construct an optical communication network having such a large capacity, high functionality, and high reliability, a technology that allows the light emitting device to freely change and control the wavelength is indispensable. In order to control the wavelength, a wavelength tunable laser incorporated in the laser apparatus is an extremely important key device.

  Many methods have been proposed as a wavelength tunable laser that satisfies these requirements.

  As one of them, a plurality of distributed feedback semiconductor lasers (DFB lasers) whose oscillation wavelengths are shifted are provided in parallel, and coarse adjustment is performed by switching between these lasers, and further fine adjustment is performed using a refractive index change due to temperature. This is described in Patent Document 1.

  However, in this structure, an optical coupler is required to couple the output ports of a plurality of lasers to one optical fiber, and as the number of parallel lasers increases, the loss at the coupler increases accordingly. . Therefore, there is a trade-off relationship between the variable wavelength range and the light output.

  On the other hand, there is an external cavity type wavelength tunable laser as a wavelength tunable laser that goes out of this trade-off and satisfies the requirements for wavelength control, and research and development are actively conducted. An external resonator type wavelength tunable laser is formed by forming a resonator using a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier) and an external reflecting mirror, and inserting a wavelength tunable reflecting structure into the resonator. Realize the selection. According to this external resonator type wavelength tunable laser, a wavelength tunable width that covers the entire C band can be obtained relatively easily.

  Since most of the basic characteristics of this type of wavelength tunable laser are determined by the wavelength tunable reflection structure, various wavelength tunable structures having excellent characteristics have been developed. For example, there is a reflection structure using an acoustic engineering filter, a dielectric filter, an etalon, or the like as disclosed in Patent Document 2 or Patent Document 3.

  There are various types of external cavity type wavelength tunable lasers configured using such a wavelength tunable structure. In order to realize a particularly high-performance light source, a configuration including a periodic channel selection filter, a wavelength tunable filter, and a reflection mirror in addition to the gain medium as shown in Patent Document 3 is effective. For example, an etalon having a periodic frequency characteristic is used as a periodic channel selection filter. An acoustic engineering filter is used as the wavelength tunable filter, and an electrically controlled wavelength tunable reflection structure or the like is used as the wavelength tunable reflection structure.

In addition, a surface-type wavelength tunable structure that can be electrically driven is effective for realizing a low-cost, high-reliability, and small-sized external resonator type tunable laser. Such a filter is, for example, as shown in Non-Patent Document 1.
JP 2003-023208 A Japanese Patent Laid-Open No. 04-69987 Japanese Patent Laid-Open No. 2000-261086 J. et al. De. Merlier et. al. , “FullC-Band External Cavity Wavelength Tunable Laser Using a Liquid-Crystal-Based Tunable Mirror, Vol. 68, Photonic Technology Letters, No. 3, IEEE PHOTICS MONTE TECH MONTE TECH TE Page (Figure 1 (a))

  However, when an external resonator type wavelength tunable laser is configured using a surface type wavelength tunable reflection structure and an attempt is made to obtain wavelength tunable characteristics, oscillation may occur at a wavelength different from the reflection spectrum peak of the wavelength tunable reflection structure used. It turns out that there is a problem.

  As a result of investigating the cause, it was found that the surface-type wavelength tunable reflection structure has a considerable degree of reflection (hereinafter referred to as residual reflection) even at wavelengths other than the reflection spectrum peak.

  When the reflection spectrum peak wavelength of the reflecting structure is λ1, the reflectance of the reflecting structure at the wavelength λ1 is R1 (%), and the reflectance of the reflecting structure at the wavelength λ2 other than the wavelength λ1 is R2 (%). When the rate difference ΔR (dB) is defined as ΔR = 10 × log (R1 / R2), there is a case where ΔR is about 8 dB.

  This problem will be described below with reference to FIG.

  The gain peak of the semiconductor optical amplifier is at the end of the wavelength tunable range (λ3 to λ4), for example, λ4, and the target wavelength λ1 is at one end of the wavelength tunable range, for example, near λ3. Consider a case where the minimum gain is obtained at this wavelength λ3 within the range.

  Because of the nature of semiconductors, semiconductor optical amplifiers have wavelength dependency in the mode gain of the amplifier when current is injected. When the external resonator type wavelength tunable laser is oscillating, the difference between the maximum mode gain and the minimum mode gain may be about 45 cm −1 as a typical value in the wavelength tunable range. When such a semiconductor optical amplifier is used as a semiconductor gain medium constituting an external cavity type wavelength tunable laser, the mirror loss of the wavelength tunable reflection structure at the wavelength λ4 indicating the maximum mode gain and the wavelength λ3 indicating the minimum mode gain. If the difference in the mirror loss of the wavelength tunable reflection structure is 45 cm −1 or less, oscillation occurs at λ4 instead of the desired oscillation wavelength λ1 (= λ3).

  When the element length of the semiconductor optical amplifier is 500 μm, which is a typical value often used, this mirror loss difference corresponds to 19.5 dB when converted to the reflectance difference ΔR of the wavelength variable reflection structure.

  That is, it can be seen that with a reflectance difference of 8 dB, the desired vicinity of the reflection spectrum peak may not be the oscillation wavelength.

  As described above, the external resonator type wavelength tunable laser using the wavelength tunable reflection structure having residual reflection has a problem that it oscillates at a wavelength other than the vicinity of the wavelength selected by the wavelength tunable reflection structure.

  This problem is particularly prominent when the wavelength variable range (λ3 to λ4) is in a range of about 35 nm or more, which is generally a full-band operation in one communication wavelength band.

  In order to solve this problem, it is only necessary to reduce the residual reflection of the wavelength tunable reflection structure. However, it is extremely difficult to set ΔR to 20 dB or more in the surface type reflection structure. Considering a possible mirror manufacturing process technology, it is often less than 16 dB.

  An object of the present invention is to reduce the above-mentioned problems and to realize a good external resonator type wavelength tunable laser even when a surface type wavelength tunable reflection structure having a considerable residual reflection is used. Another object of the present invention is to provide a semiconductor optical amplifier that realizes a semiconductor gain region as a component in order to realize the wavelength tunable laser.

  An external resonator type wavelength tunable laser according to the present invention is an external resonator type wavelength tunable laser having a surface type reflection structure capable of varying a reflection spectrum peak wavelength and including at least a semiconductor gain region. When the spectral peak wavelength is an arbitrary wavelength λ1 included in the wavelength tunable range of the wavelength tunable laser and the wavelength indicating the maximum gain of the semiconductor gain region at the time of laser oscillation is λ2, the loss at the wavelength λ1 of the reflective structure And the absolute value ΔLoss (dB) of the difference between the loss at the wavelength λ2 is larger than the difference Δg (dB) between the gain at the wavelength λ1 and the gain at the wavelength λ2 of the semiconductor gain region at the time of laser oscillation. A semiconductor gain region having a product ΓL of a confinement constant Γ and a semiconductor gain region length L is incorporated.

  Furthermore, in the external resonator type wavelength tunable laser, the reflectance of the reflective structure at the wavelength λ1 is R1 (%), the reflectance of the reflective structure at the wavelength λ2 is R2 (%), and the reflectance When the difference ΔR (dB) is ΔR = 10 × log (R1 / R2), ΔR is larger than 10 dB.

  Further, in the external resonator type wavelength tunable laser, the variable range of the reflection spectrum peak of the reflection structure is 40 nm from the wavelength (λ4-40) nm to the wavelength λ4, and one round of the external resonator type wavelength tunable laser. And the active region of the semiconductor gain region is composed of multiple quantum wells, and Γ, L, and ΔR according to claim 1 are 25 μm <Γ × L <35 μm, and , 6 dB <ΔR <16 dB, the gain peak wavelength λ0 at the time of carrier density injection having a maximum mode gain equal to the internal loss of the semiconductor gain region is larger than −1.45 × ΔR + (λ4 + 16), and A semiconductor gain region smaller than (−0.14 × (ΓL) +7.79) × ΔR + (− 0.82 × (ΓL) + λ4 + 20) is incorporated.

  Further, the external resonator type wavelength tunable laser has a semiconductor gain region having Γ and L such that the oscillation threshold current density Jth is 30 kA / cm 2 or less.

  The semiconductor optical amplifier may be integrated with a passive region capable of changing the phase of light in the resonator of the external resonator type tunable laser.

Furthermore, the waveguide near the end face on the external resonator side of the semiconductor optical amplifier may not be perpendicular to the end face.
(Function)
The external cavity type wavelength tunable laser disclosed in the present invention includes a semiconductor gain medium, and is configured by a surface-type reflection structure capable of changing the reflection spectrum peak wavelength.

  As described above, the surface type reflection structure generally has residual reflection, and it is extremely difficult to set the reflectance difference ΔR of the wavelength variable reflection structure to 20 dB or more. Therefore, the present invention is also effective in the range of less than 20 dB.

  The principle of the present invention will be described in detail with reference to FIG.

  The external resonator type laser oscillates at the wavelength when the loss in the external resonator is equal to the gain of the semiconductor gain medium. When the wavelength tunable reflecting structure is used for the external reflecting mirror, the loss of the external resonator varies depending on the wavelength, and the loss becomes the lowest at the wavelength selected by the reflecting structure. This is because the reflectance at the selected wavelength in the reflective structure is the highest, and the amount of light fed back to the resonator is also increased.

  In the external cavity type wavelength tunable laser using the wavelength tunable reflection structure with ΔR of less than 20 dB, the absolute value ΔLoss of the difference between the loss at the wavelength λ1 and the loss at the wavelength λ2 selected in the wavelength tunable reflection structure is the wavelength λ1 and By using the semiconductor gain medium that is larger than the gain difference Δg of the semiconductor gain medium at the wavelength λ2, it is possible to oscillate stably at the desired wavelength λ1 without oscillating at the wavelength λ2.

  In general, the gain curve of the semiconductor gain medium built in the external resonator type tunable laser reaches a current density that gives the same gain as the configured external resonator loss, that is, the oscillation threshold current density Jth, and is fixed when the laser oscillates. Is done. The fact that Δg at that time is small, that is, that the gain curve is smooth, is important for oscillation at the desired wavelength λ1.

  When a semiconductor optical amplifier having an active layer having a general waveguide structure is used as the semiconductor gain medium, there are an optical confinement coefficient Γ and a gain medium length L as parameters indicating the characteristics of the semiconductor optical amplifier. In that case, it was found that the product of these coefficients (hereinafter referred to as ΓL) can substantially represent the wavelength dependence of the gain of the active layer. That is, Δg can be controlled by ΓL. By using a semiconductor optical gain medium having an appropriate ΓL, an external resonator type tunable laser capable of oscillating at a desired wavelength can be realized.

  In the active layer structure having a large ΓL, since the current density for producing the same gain may be small, oscillation occurs with a small current density. In that case, the gain curve becomes steep. That is, since Δg becomes large, the problem is that Δg exceeds ΔLoss and does not oscillate at a desired wavelength.

  On the other hand, in the active layer structure having a small ΓL, the current density is increased in order to obtain a gain that exceeds the loss of the resonator of the external cavity type tunable laser. In this case, Δg is small, which is desirable in terms of oscillation at a desired wavelength λ1. However, if the current density Jth reaching the threshold is excessively high, the influence of the heat generation and leakage current of the device appears to be remarkable, and the light output is saturated with respect to current injection, and the maximum light output decreases. In addition, there is a problem that the element life is shortened. In general, it is known that this problem occurs unless Jth is 30 kA / cm 2 or less. By setting Jth to 30 kA / cm 2 or less, it is possible to obtain an effect of sufficiently extending the element lifetime of the wavelength tunable light source.

  The gain peak wavelength λ0 of the light emitting layer when it has the maximum mode gain equal to the internal loss of the semiconductor optical amplifier is also an important parameter for determining the absolute wavelength tunable range of the external resonator type tunable laser that constitutes the semiconductor optical amplifier. It is.

  In FIG. 3, when ΔR is 10 dB and the optical loss in one round of the external cavity type wavelength tunable laser is 20 dB to 25 dB, the C band used in optical communication, so-called 1530 nm to 1570 nm, is used as the wavelength tunable range. The calculation result about the characteristic of the semiconductor gain medium which should be satisfied is shown.

  As shown in FIG. 3, it was found that ΓL and λ0 necessary for obtaining a wavelength variable range from 1530 nm to 1570 nm are included in the hatched range. According to this, it can be seen that when ΓL is in a practical range of 20 microns to 40 microns, at least the allowable λ0 range is 10 nm. This wavelength range is considered to be an allowable range that can be realized with a sufficient yield according to the current MQW manufacturing technology. Therefore, an effect of maintaining a high element manufacturing yield can be obtained by using a planar reflection structure having a ΔR larger than 10 dB.

  If the allowable wavelength range is narrower than this range, for example, the emission wavelength of the growth layer is set within this wavelength due to variations in the production batch of the growth layer produced by MOVPE on an InP substrate having a size of 2 inches, in-plane distribution, etc. Since it becomes difficult to maintain, the yield of device fabrication is reduced.

  When ΔR is in the range of 10 dB or less, in order to ensure a certain wavelength tunable range (for example, the above-mentioned 40 nm), it is necessary to realize the value of ΓL and λ0 within a certain range. . Specific examples are shown below.

  In order to realize an external cavity type wavelength tunable laser having a wavelength range of 1530 nm to 1570 nm using a wavelength tunable reflection structure of ΔR = 8 dB, an active layer mounted on the external cavity type wavelength tunable laser is configured with MQW. FIG. 4 shows the result of concrete calculation of the range of values of Γ, L, and λ0 necessary for satisfying the semiconductor optical amplifier.

  The upper limit λ0, max and the lower limit λ0, min of the range shown in FIG. 4 are qualitatively described as follows. When the gain peak wavelength of the SOA at the time of laser oscillation is longer than the peak wavelength of the wavelength tunable reflection structure, it is limited by λ0, max. As ΓL increases, Δg increases with respect to ΔLoss, as described above, and the allowable wavelength range decreases.

  Conversely, when the SOA gain peak wavelength is on the shorter wavelength side than the peak wavelength of the wavelength tunable reflection structure, the SOA is limited to λ0, min. In this case, the gain of the semiconductor optical amplifier near the peak wavelength of the wavelength tunable reflection structure is less changed by current injection, so the value of λ0, min does not greatly depend on the amount of change in L.

  In the case of ΔR = 6 dB, the result of the same calculation is shown in FIG. As described above, the allowable ranges of ΓL and λ0 are extremely narrow, and when ΔR is smaller than this, it is difficult to realize a desired wavelength variable characteristic.

  In addition to the above results, from the results of FIG. 6 calculated for the case of ΔR = 16 dB and the calculation results of ΔR = 10 dB shown in FIG. 3, λ0, max and λ0, min are within the range of ΔR. It can be extracted at each ΓL. The results are shown in FIGS.

  Based on these calculation results, if the longest wavelength of the variable range of the external resonator type tunable laser is λ4, the semiconductor optical amplifier built in the external resonator type tunable laser has a variable reflection spectrum peak of the reflection structure. The range is 40 nm from the wavelength (λ4-40) nm to the wavelength λ4, the cavity loss of one round of the external cavity type tunable laser is 20 to 25 dB, and the active region of the semiconductor gain region is a multiquantum The maximum mode equal to the internal loss of the semiconductor gain region when Γ, L, and ΔR according to claim 1 satisfy 25 μm <Γ × L <35 μm and 6 dB <ΔR <16 dB. The gain peak wavelength λ0 at the time of injection of carrier density with gain is larger than −1.45 × ΔR + (λ4 + 16), and (−0.14 × (ΓL) +7.7. ) × ΔR + (− 0.82 × (ΓL) + λ4 + 20) satisfying that the range is smaller than that of the external resonator type wavelength tunable laser even if the surface type light reflecting structure having residual reflection is used. It can be said that it can be realized.

  If these semiconductor optical amplifiers are integrated with a passive region that can change the phase of the light in the resonator of the external cavity type tunable laser, current can be injected or voltage can be applied to the passive region. Because the Fabry-Perot mode by an external resonator can be accurately adjusted to the desired wavelength, it is possible to accurately match the wavelength to the wavelength grid determined by the optical communication system. The laser can have higher performance.

  Further, in the semiconductor optical amplifier, the waveguide near the end face on the external resonator side may not be perpendicular to the end face. In the external resonator type laser, if there is a reflection point inside, a composite resonator is formed, so that it becomes difficult to control the laser oscillation wavelength. If the waveguide in the vicinity of the end face on the external resonator side is not perpendicular to the end face, the reflectivity at the end face can be lowered, so that higher performance can be achieved.

  The first effect is that in the external cavity type wavelength tunable laser having a surface type reflection structure capable of changing the reflection spectrum peak wavelength and including at least a semiconductor gain region, the reflection spectrum peak wavelength of the reflection structure is The wavelength λ1 included in the wavelength tunable range of the wavelength tunable laser, the wavelength indicating the maximum gain of the semiconductor gain region during laser oscillation is λ2, and the loss at the wavelength λ1 and the loss at the wavelength λ2 of the reflection structure Is the product ΓL of the optical confinement constant Γ and the semiconductor gain region length L that are larger than the difference Δg (dB) between the gain at the wavelength λ2 and the gain at the wavelength λ2 at the time of laser oscillation. By incorporating a semiconductor gain region having It is possible to provide an external resonator type wavelength variable laser that performance be used.

  The second effect is that the reflection structure of the surface type reflection structure incorporated in the external resonator type wavelength tunable laser has a reflectance of R1 (%) at the wavelength λ1, and the reflection structure of the reflection structure at the wavelength λ2. When the reflectivity is R2 (%) and the reflectivity difference ΔR (dB) is ΔR = 10 × log (R1 / R2), by making ΔR larger than 10 dB, the manufacturability, that is, the device manufacturing yield. The effect of maintaining high can be obtained.

  The third effect is that an element lifetime of the wavelength tunable light source is obtained by incorporating a semiconductor gain region having Γ and L such that the oscillation threshold current density Jth of the external resonator type wavelength tunable laser is 30 kA / cm 2 or less. The effect of making the length sufficiently long can be obtained.

  The fourth effect is that these semiconductor optical amplifiers are configured to integrate a passive region capable of changing the phase of light in the resonator of the external resonator type wavelength tunable laser, thereby providing an external resonator. Fabry-Perot mode can be precisely adjusted to the desired wavelength, and the wavelength can be accurately adjusted to the wavelength grid defined in the optical communication system, providing a higher-performance external cavity tunable laser can do.

  The fifth effect is that it is difficult to control the laser oscillation wavelength by making the waveguide in the vicinity of the end face on the external resonator side of these semiconductor optical amplifiers not perpendicular to the end face. It is possible to reduce the unnecessary reflectivity inside one external resonator, and it is possible to provide a high-performance external resonator type wavelength tunable laser with easier wavelength control.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 9 is a diagram showing a configuration of an external resonator type tunable laser according to the best embodiment.

  The external resonator type variable laser has, as a basic configuration, a semiconductor element 1 that is a semiconductor gain region and a wavelength tunable filter 2 that is a surface type reflection structure capable of changing a reflection spectrum peak wavelength. The semiconductor element 1 includes a semiconductor optical amplifier 3 that is an active element and a phase adjustment region 4 that is a passive element. The semiconductor element 1 has the semiconductor optical amplifier 3 side as the light output side, and a low reflection coating 5 (reflectance of 1% to 10%) is applied to the end face thereof. Moreover, the semiconductor element 1 has the phase adjustment region 4 side as the external resonator side, and an antireflection coating 6 (reflectance of 1% or less) is applied to the end face. The phase adjustment region 4 side may be the light output side.

  The semiconductor optical amplifier 3 is composed of multiple quantum wells (MQW), and generates and amplifies light by performing current injection.

  The phase adjustment region 4 is composed of a bulk composition or multiple quantum wells, and has a wide band gap that does not absorb laser oscillation light. In the phase adjustment region 4, the refractive index is changed by current injection or voltage application, and the phase of the laser can be changed.

  The semiconductor optical amplifier 3 and the phase adjustment region 4 are electrically separated sufficiently so that they do not interfere with each other. Specifically, the semiconductor optical amplifier 3 and the phase adjustment region 4 are separated by a separation resistor of 1 kilohm or more.

  A wavelength tunable reflection structure 2 is disposed on the external resonator side of the semiconductor element 1. A collimating lens 7 is disposed between the wavelength variable reflection structure 2 and the semiconductor element 1. The collimating lens 7 converts the light beam from the semiconductor element 1 into parallel light. The beam collimated by the collimator lens 7 is then reflected by the wavelength tunable reflection structure 2 and fed back to the original semiconductor element 1.

  In the external resonator type wavelength tunable laser configured as described above, by applying the technique disclosed in this patent to the semiconductor optical amplifier 3 included in the semiconductor element 1, a surface type reflection structure capable of changing the reflection spectrum peak wavelength. Even if the wavelength tunable reflection structure is used, at least one wavelength tunable range of C band and L band in optical fiber communication can be provided.

  In general, an external resonator type wavelength tunable laser configured by a wavelength tunable reflection structure which is a surface type reflection structure capable of varying a reflection spectrum peak wavelength, where ΔR <20 dB, and a wavelength λ1 in the vicinity of the reflection spectrum peak of the wavelength tunable reflection structure The condition to be satisfied by the semiconductor optical amplifier in order to oscillate at is that the absolute value ΔLoss of the difference between the loss at the wavelength λ1 and the loss at the wavelength λ2 of the wavelength tunable reflection structure is the gain of the semiconductor optical amplifier at the wavelength λ1 and the semiconductor at the wavelength λ2. The difference is to be larger than the difference Δg with the gain of the optical amplifier.

  In order to realize this, it is necessary to first determine the dependence of the gain of MQW on the injection current density that is expected from the active layer material used as the active layer or actually manufactured and measured. Based on this result, the range of values of ΓL and λ0 to be satisfied by the semiconductor optical amplifier is derived from the mirror loss of the external cavity type tunable laser and the operating wavelength range of the target tunable laser.

  FIG. 6 shows the ranges of ΓL and λ0 for obtaining the wavelength variable range of the C band (1530 to 1570 nm) when ΔR = 16 dB.

  The range obtained similarly in the case of ΔR = 10 dB is shown in FIG.

  The range similarly obtained in the case of ΔR = 8 dB is shown in FIG.

  The range similarly obtained when ΔR = 6 dB is shown in FIG.

  In the case of other values of ΔR, the range of λ0 is a value between λ0, min and λ0, max calculated in FIGS.

  A semiconductor optical amplifier having ΓL and λ0 included in this range may be used according to the value of ΔR of the wavelength tunable filter to be used.

  In the present embodiment, it is possible to accurately match the wavelength to the wavelength grid determined by the optical communication system by making the laser oscillation wavelength completely coincide with the maximum reflection peak wavelength of the wavelength variable reflection structure 7. It is. Matching the oscillation wavelength is realized by changing the refractive index by passing a current through the phase adjustment region 4 provided in the external resonator.

In the present embodiment, the reason why the reflectivity of the low-reflection coating 4 on the light output side is set to 1 to 10% is that when the reflectivity is less than 1%, the laser threshold value is increased to increase the light output. This is because when the reflectance is higher than 10%, the transmittance of the low-reflection coating 4 is less than 90%, and a high light output cannot be obtained.
(Description of manufacturing method)
Next, the manufacturing method of the best embodiment will be described.

  First, a method for manufacturing the semiconductor element 1 will be described. In the semiconductor element 1, a semiconductor optical amplifier 3 and a phase adjustment region 4 are monolithically integrated. For this integration, a known butt joint technique may be used, or a known selective growth technique may be used. When using the butt joint technology, for example, the following manufacturing method is performed.

  First, an MQW structure used as a semiconductor optical amplifier is grown. At this time, the growth is performed so as to obtain an MQW structure satisfying the appropriate ΓL and λ0 based on the residual reflectance ΔR of the wavelength tunable reflection structure to be used. The guided mode is calculated from the optical confinement structure, and Γ may be calculated. When MQW is used as the active layer, it is often about 0.007 to 0.009 per layer, and can be approximated by that value. Usually, about 3 to 10 can be used as the number of well layers. When growing MQW, the absolute wavelength λ0 of the gain of this MQW active layer is estimated based on the peak wavelength of normal PL (photoluminescence). Since the PL wavelength varies depending on the intensity of the excitation light source of the measurement system and the emission efficiency of MQW, it is necessary to investigate the correlation with λ0 in advance. Usually, the PL wavelength is often shorter by about 20 to 30 nm.

  In this way, a part of the fabricated MQW structure is removed by wet etching or dry etching, and an optical waveguide structure that newly becomes a phase adjustment region is grown. In this structure, it is necessary to shorten the wavelength sufficiently from the light emitted from the semiconductor optical amplifier and reduce the absorption in this region. The structure may be a bulk structure or an MQW structure.

  Note that the MQW structure of the semiconductor optical amplifier may be grown after first growing the phase adjustment region and removing unnecessary portions.

  Thereafter, unnecessary regions are removed by leaving a region to be an optical waveguide by dry etching or wet etching using a dielectric such as SiO 2 as a mask. The width of the optical waveguide is preferably about 0.5 μm to 2 μm. At this time, a region for recombining electrons and holes leaked to the block structure may be left.

  Next, a pnp block structure of InP is grown, and p-InP cladding growth is performed as necessary.

  After the growth, the semiconductor optical amplifier and the phase adjustment region are electrically separated by removing a layer having a small electric resistance by etching or the like. Next, after depositing a dielectric such as SiO2, a current injection region is formed by etching or the like. After that, a p-electrode is deposited, and unnecessary electrodes are removed. Then, the substrate is thinned by polishing, and an n-electrode is deposited on the back surface.

  The semiconductor element is cut into a bar state by cleavage, that is, a state in which a plurality of semiconductor elements are connected horizontally. At that time, the ΓL is cut so as to have a resonator length L satisfying a desired value, and the cleavage end face is coated. Usually, the length of L is often about 200 to 1000 μm. When the semiconductor optical amplifier 3 side is the light output side, a low reflection coating 5 is performed on the end face by depositing a dielectric multilayer film. When the opposite phase adjustment region 4 side is the external resonator side, the antireflection coating 6 is performed by depositing another dielectric multilayer film on the end face.

  Thereafter, the semiconductor elements are divided one by one and mounted on the subcarrier together with the variable wavelength reflection structure 2 and the collimating lens 7. At this time, the semiconductor element is arranged accurately so that a current flows to emit light and the light beam travels linearly.

The external cavity type wavelength tunable laser can be manufactured through the above steps.
(Another embodiment of the invention)
Even when the wavelength tunable reflection structure function is realized by a configuration group other than those described above, the effect of the present invention can be similarly expected when a surface type reflection structure capable of changing the reflection spectrum peak wavelength is used.

  Although the C band (1530 to 1570 nm) has been described as the wavelength variable range, this can be similarly applied to optical communication in other wavelength bands, for example, the S band and the L band.

  The phase adjustment region 4 integrated in the semiconductor element 1 does not need to be monolithically integrated with the semiconductor optical amplifier, and may exist in the external resonator as an independent component, or may be a semiconductor optical amplifier or a wavelength tunable reflection structure. It is also possible to realize this by slightly moving the lens with a known piezo element or the like.

  In the semiconductor optical amplifier, the waveguide near the end face on the external resonator side may not be perpendicular to the end face. In the external resonator type laser, if there is a reflection point inside, a composite resonator is formed, so that it becomes difficult to control the laser oscillation wavelength. If the waveguide in the vicinity of the end face on the external resonator side is not perpendicular to the end face, the reflectivity at the end face can be lowered, so that higher performance can be achieved.

  A first embodiment of the present invention will be described below with reference to the drawings.

  FIG. 10 is a diagram showing the configuration of an external resonator type wavelength tunable laser device according to the first embodiment of the present invention.

  The external resonator type wavelength tunable laser device of this embodiment has a wavelength in which the semiconductor element 1 including the semiconductor optical amplifier 3, the collimating lens 7, the surface type wavelength selection filter 8, and the C band (1530 to 1570 nm) are in the operating range. The variable reflection structure 2 is used. ΔR of this wavelength tunable reflection structure is 8 dB.

  In the semiconductor element 1, a semiconductor optical amplifier 3 and a phase adjustment region 4 are monolithically integrated. This integration was made using butt joint technology.

  First, an MQW structure used as a semiconductor optical amplifier was grown with a growth pressure of 98.6 kPa and a growth temperature of 625 ° C., and six MQW layers (a well has a 1.58 μm composition compression strained InGaAsP and a barrier has a 1.2 μm composition InGaAsP). SCH (separate confinement heterostructure) is arranged before and after this layer, and Γ of the well region is 0.05. By setting the resonator length L to 600 μm, ΓL can be set to 30 μm. From prior studies, it is known that the difference between the PL wavelengths λPL and λ0 obtained from this MQW structure is λ0−λPL = 26 nm. Therefore, growth was performed so that the PL wavelength of the MQW layer was 1560 nm.

  By doing in this way, the semiconductor optical amplifier of the shaded part shown in FIG. 4 can be obtained.

  Next, a part of the fabricated MQW structure was removed by dry etching, and an optical waveguide structure that newly became a phase adjustment region was grown under conditions of a growth pressure of 98.6 kPa and a growth temperature of 625 ° C. The optical waveguide structure has a bulk structure as a core, has a thickness of 170 nm, and a composition with a PL wavelength of 1.3 μm.

  After that, a dielectric such as SiO2 is deposited, and a trench having a depth of 2 μm is etched by dry etching using a stripe portion serving as an optical waveguide and a portion where electrons and holes leaking to the block structure are recombined as a mask. Removed. The width of the optical waveguide was 1.5 μm.

  Next, a pnp block structure having a thickness of 2 μm is grown by InP, and after removing the SIO2 mask, a p-InP cladding layer having a thickness of 2 μm and a p + -InGaAs contact layer having a thickness of 0.3 μm are grown by growing the entire surface. Grown at 3 kPa. Thereafter, in order to electrically isolate the semiconductor optical amplifier and the phase adjustment region, the contact layer is removed, a SiO2 film is formed on the entire surface, a current injection window is formed, and a Cr / Au upper p-electrode is deposited by sputtering. Then, the electrodes were separated for each region by patterning. After polishing the substrate, an AuGeNi lower electrode was formed by sputtering. Thereafter, cleavage was performed so that the semiconductor optical amplifier region was 600 μm and the phase adjustment region was 200 μm. Finally, in order to set the semiconductor optical amplifier 3 side to the light output side, a low reflection coating 5 was performed by depositing a dielectric multilayer film on the end face. In order to set the opposite phase adjustment region 4 side to the external resonator side, the antireflection coating 6 was performed by depositing another dielectric multilayer film on the end face.

  Thereafter, the semiconductor elements are divided one by one and mounted on the subcarrier 9 together with the variable wavelength reflection structure 2, the collimating lens 7, and the wavelength selection filter 8. At that time, the semiconductor element was caused to emit light by passing an electric current, and was arranged accurately so that the light beam traveled linearly, and an external resonator type wavelength tunable laser was manufactured.

  This external cavity type wavelength tunable laser can vary the wavelength between 1525 nm and 1568 nm, and oscillates at a threshold of 20 mA or less in all wavelength regions. By injecting a current of about 8 mA into the phase adjustment region, the light intensity could be adjusted to the maximum at the wavelength determined by the optical communication system. Further, the maximum light output was as good as 32 mW as the fiber coupled light output, and a sufficient wavelength variable function could be obtained by the method of the present invention.

  A second embodiment of the present invention will be described below with reference to the drawings.

  FIG. 11 is a diagram showing a configuration of an external resonator type wavelength tunable laser device according to a second embodiment of the present invention.

  The external resonator type wavelength tunable laser device of this embodiment includes a semiconductor element 1 including a semiconductor optical amplifier 3, a collimating lens 7, a wavelength selection filter 8, and a wavelength tunable filter having an L band (1570 to 1610) nm operating range. 10 and a total reflection mirror 11. ΔR of the wavelength tunable reflecting structure formed by the wavelength tunable filter and the total reflection mirror is 6 dB.

  In the semiconductor element 1, a semiconductor optical amplifier 3 and a phase adjustment region 4 are monolithically integrated. This integration was made using butt joint technology.

  First, an MQW structure used as a semiconductor optical amplifier was grown at a growth pressure of 98.6 kPa, a growth temperature of 625 ° C., and eight MQW layers (a well has a 1.62 μm composition compression strained InGaAsP and a barrier has a 1.24 μm composition InGaAsP). SCH is arranged before and after this layer, and Γ of the well region is 0.067. By setting the resonator length L to 400 μm, ΓL can be set to 26.8 μm. From prior studies, it is known that the difference between the PL wavelengths λPL and λ0 obtained from this MQW structure is λ0−λPL = 26 nm. Therefore, the MQW layer was grown so that the PL wavelength was 1600 nm.

  By doing in this way, the semiconductor optical amplifier of the shaded part shown in FIG. 5 can be obtained. The absolute value of the wavelength may be shifted +40 nm with respect to the range shown in FIG.

  Next, a part of the fabricated MQW structure was removed by dry etching, and an optical waveguide structure that newly became a phase adjustment region was grown under conditions of a growth pressure of 98.6 kPa and a growth temperature of 625 ° C. The optical waveguide structure has a bulk structure as a core, has a thickness of 170 nm, and a composition with a PL wavelength of 1.3 μm.

  After that, a dielectric such as SiO2 is deposited, and a trench having a depth of 2 μm is etched by dry etching using a stripe portion serving as an optical waveguide and a portion where electrons and holes leaking to the block structure are recombined as a mask. Removed. The width of the optical waveguide was 1.5 μm.

  Next, a pnp block structure having a thickness of 2 μm is grown by InP, and after removing the SIO2 mask, a p-InP cladding layer having a thickness of 2 μm and a p + -InGaAs contact layer having a thickness of 0.3 μm are grown by growing the entire surface. Grown at 3 kPa. Thereafter, in order to electrically isolate the semiconductor optical amplifier and the phase adjustment region, the contact layer is removed, a SiO2 film is formed on the entire surface, a current injection window is formed, and a Cr / Au upper p-electrode is deposited by sputtering. Then, the electrodes were separated for each region by patterning. After polishing the substrate, an AuGeNi lower electrode was formed by sputtering. Thereafter, cleavage was performed so that the semiconductor optical amplifier region was 400 μm and the phase adjustment region was 150 μm. Finally, in order to set the semiconductor optical amplifier 3 side to the light output side, a low reflection coating 5 was performed by depositing a dielectric multilayer film on the end face. In order to set the opposite phase adjustment region 4 side to the external resonator side, the antireflection coating 6 was performed by depositing another dielectric multilayer film on the end face.

  Thereafter, the semiconductor elements are divided one by one and mounted on the subcarrier 9 together with the wavelength tunable filter 10, the total reflection mirror 11, the collimator lens 7, and the wavelength selection filter 8. At that time, the semiconductor element was caused to emit light by passing an electric current, and was arranged accurately so that the light beam traveled linearly, and an external resonator type wavelength tunable laser was manufactured.

  This external cavity type wavelength tunable laser can vary the wavelength between 1575 nm and 1615 nm, and oscillates at a threshold of 25 mA or less in all wavelength regions. By injecting a current of about 10 mA into the phase adjustment region, the light intensity could be adjusted to the maximum at the wavelength determined by the optical communication system. Further, the maximum light output was 30 mW as a fiber coupled light output, and a satisfactory wavelength variable function could be obtained by the method of the present invention.

  A third embodiment of the present invention will be described below with reference to the drawings.

  FIG. 12 is a diagram showing a configuration of an external resonator type wavelength tunable laser device according to a third embodiment of the present invention.

  The external resonator type wavelength tunable laser device of this embodiment includes a semiconductor element 1 including a semiconductor optical amplifier 3, a collimating lens 7, a wavelength selection filter 8, and a wavelength tunable reflection structure having an operating range of C band (1530 to 1570 nm). 2. ΔR of this variable wavelength reflection structure is 8 dB.

  In the semiconductor element 1, the semiconductor optical amplifier 3 and the phase adjustment region 4 are monolithically integrated, and the light emitting ends of both regions are inclined by 7 degrees with respect to the end face. This integration was made using butt joint technology.

  First, an MQW structure used as a semiconductor optical amplifier was grown at a growth pressure of 98.6 kPa and a growth temperature of 625 ° C., with eight MQW layers (well: 1.58 μm composition compression strained InGaAsP, barrier: 1.2 μm composition InGaAsP). SCH is arranged before and after this layer, and Γ of the well region is 0.066. By setting the resonator length L to 450 μm, ΓL can be set to 30 μm. From prior studies, it is known that the difference between the PL wavelengths λPL and λ0 obtained from this MQW structure is λ0−λPL = 26 nm. Therefore, growth was performed so that the PL wavelength of the MQW layer was 1560 nm.

  By doing in this way, the semiconductor optical amplifier of the shaded part shown in FIG. 4 can be obtained.

  Next, a part of the fabricated MQW structure was removed by dry etching, and an optical waveguide structure that newly became a phase adjustment region was grown under conditions of a growth pressure of 98.6 kPa and a growth temperature of 625 ° C. The optical waveguide structure has a bulk structure as a core, has a thickness of 170 nm, and a composition with a PL wavelength of 1.3 μm.

  After that, a dielectric such as SiO2 is deposited, and a trench having a depth of 2 μm is etched by dry etching using a stripe portion serving as an optical waveguide and a portion where electrons and holes leaking to the block structure are recombined as a mask. Removed. The width of the optical waveguide is 1.5 μm, and the waveguide is smoothly bent with a radius of curvature of 1 mm so that the waveguide is inclined 7 degrees from the end face at the cleavage position.

  Next, a pnp block structure having a thickness of 2 μm is grown by InP, and after removing the SIO2 mask, a p-InP cladding layer having a thickness of 2 μm and a p + -InGaAs contact layer having a thickness of 0.3 μm are grown by growing the entire surface. Grown at 3 kPa. Thereafter, in order to electrically isolate the semiconductor optical amplifier and the phase adjustment region, the contact layer is removed, a SiO2 film is formed on the entire surface, a current injection window is formed, and a Cr / Au upper p-electrode is deposited by sputtering. Then, the electrodes were separated for each region by patterning. After polishing the substrate, an AuGeNi lower electrode was formed by sputtering. Thereafter, cleavage was performed so that the semiconductor optical amplifier region was 450 μm and the phase adjustment region was 200 μm. Finally, in order to set the semiconductor optical amplifier 3 side to the light output side, a low reflection coating 5 was performed by depositing a dielectric multilayer film on the end face. In order to set the opposite phase adjustment region 4 side to the external resonator side, the antireflection coating 6 was performed by depositing another dielectric multilayer film on the end face.

  Thereafter, the semiconductor elements are divided one by one and mounted on the subcarrier 9 together with the variable wavelength reflection structure 2, the collimating lens 7, and the wavelength selection filter 8. At that time, the semiconductor element was caused to emit light by passing an electric current, and was arranged accurately so that the light beam traveled linearly, and an external resonator type wavelength tunable laser was manufactured.

  This external cavity type wavelength tunable laser can vary the wavelength between 1527 nm and 1570 nm, and oscillates with a threshold of 20 mA or less in all wavelength regions. By injecting a current of about 8 mA into the phase adjustment region, the light intensity could be adjusted to the maximum at the wavelength determined by the optical communication system. Further, the maximum light output was 30 mW as a fiber coupled light output, and a satisfactory wavelength variable function could be obtained by the method of the present invention.

  As an application example of the present invention, there is a medium and long distance light source for wavelength multiplexing communication used for a trunk line system and an access system.

It is a figure which shows the relationship between the wavelength variable reflection structure loss and the gain of a semiconductor optical amplifier which may arise when there exists a residual reflection. In the wavelength tunable laser incorporating the semiconductor optical amplifier according to the present invention, it is a diagram showing the relationship between the wavelength tunable reflection structure loss and the gain of the semiconductor optical amplifier. When ΔR = 10 dB and the optical loss in one round of the external resonator type wavelength tunable laser is 20 dB to 25 dB, the calculation result is shown for the conditions of the semiconductor optical amplifier that should be satisfied to obtain 1530 nm to 1570 nm as the wavelength variable range. is there. When ΔR = 8 dB and the optical loss in one round of the external resonator type wavelength tunable laser is 20 dB to 25 dB, the calculation result is shown for the condition of the semiconductor optical amplifier that should be satisfied to obtain 1530 nm to 1570 nm as the wavelength variable range. is there. When ΔR = 6 dB and the optical loss in one round of the external cavity type wavelength tunable laser is 20 dB to 25 dB, the calculation result is shown for the conditions of the semiconductor optical amplifier that should be satisfied to obtain 1530 nm to 1570 nm as the wavelength variable range. is there. When ΔR = 16 dB and the optical loss in one round of the external resonator type wavelength tunable laser is 20 dB to 25 dB, the calculation result is shown for the condition of the semiconductor optical amplifier that should be satisfied to obtain 1530 nm to 1570 nm as the wavelength variable range. is there. FIG. 6 is a diagram in which ΔR dependence of the maximum value λ0, max of λ0 allowed is extracted using ΓL as a parameter. FIG. 6 is a diagram in which ΔR dependence of a minimum value λ0, min of λ0 allowed is extracted using ΓL as a parameter. It is a figure which shows the structure of the external resonator type | mold wavelength-variable laser by the best form for implementing this invention. It is a figure which shows the structure of the external resonator type | mold wavelength-variable laser in the 1st Embodiment of this invention. It is a figure which shows the structure of the external resonator type | mold wavelength-tunable laser in the 2nd Embodiment of this invention. It is a figure which shows the structure of the external resonator type | mold wavelength-tunable laser in the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor element 2 Wavelength variable reflection structure 3 Semiconductor optical amplifier 4 Phase adjustment area | region 5 Low reflection coating film 6 Antireflection coating film 7 Collimating lens 8 Wavelength selection filter 9 Subcarrier 10 Wavelength variable filter 11 Total reflection mirror

Claims (6)

In an external cavity type wavelength tunable laser having a surface type reflection structure capable of changing the reflection spectrum peak wavelength and including at least a semiconductor gain region,
When the reflection spectrum peak wavelength of the reflection structure is an arbitrary wavelength λ1 included in the wavelength variable range of the wavelength tunable laser, and the wavelength indicating the maximum gain of the semiconductor gain region at the time of laser oscillation is λ2, The absolute value ΔLoss (dB) of the difference between the loss at wavelength λ1 and the loss at wavelength λ2 is larger than the difference Δg (dB) between the gain at wavelength λ1 and the gain at wavelength λ2 in the semiconductor gain region during laser oscillation. An external cavity type wavelength tunable laser comprising a semiconductor gain region having a product ΓL of an optical confinement constant Γ and a semiconductor gain region length L. In the external cavity type tunable laser,
The reflectance of the reflective structure at the wavelength λ1 is R1 (%), the reflectance of the reflective structure at the wavelength λ2 is R2 (%), and the reflectance difference ΔR (dB) is ΔR = 10 × log ( 2. The external cavity type wavelength tunable laser according to claim 1, wherein ΔR is larger than 10 dB when R1 / R2). In the external cavity type tunable laser,
The variable range of the reflection spectrum peak of the reflection structure is 40 nm from a wavelength (λ4-40) nm to a wavelength λ4,
The round-trip resonator loss of the external resonator type tunable laser is 20 to 25 dB,
The active region of the semiconductor gain region is composed of multiple quantum wells,
The reflectance of the reflective structure at the wavelength λ1 is R1 (%), the reflectance of the reflective structure at the wavelength λ2 is R2 (%), and the reflectance difference ΔR (dB) is ΔR = 10 × log ( R1 / R2)
When the relationship between the optical confinement constant Γ, the semiconductor gain region length L, and the reflectance difference ΔR satisfy 25 μm <Γ × L <5 μm and 6 dB <ΔR <16 dB, the internal loss of the semiconductor gain region The gain peak wavelength λ0 at the time of carrier density injection having the same maximum mode gain is larger than −1.45 × ΔR + (λ4 + 16), and (−0.14 × (ΓL) +7.79) × ΔR + (− 0) 2. The external cavity type wavelength tunable laser according to claim 1, wherein a semiconductor gain region smaller than .82 × (ΓL) + λ4 + 20) is incorporated.   4. The semiconductor gain region according to claim 1, further comprising: a semiconductor gain region having the optical confinement constant Γ and the semiconductor gain region length L such that an oscillation threshold current density Jth is 30 kA / cm 2 or less. An external resonator type wavelength tunable laser described in 1.   2. A semiconductor optical amplifier in which a passive region capable of changing a phase of light in a resonator of the external cavity type tunable laser is integrated as the semiconductor gain region. 5. The external resonator type wavelength tunable laser according to any one of 4.   6. The external of claim 1, wherein the semiconductor gain medium includes a semiconductor optical amplifier in which the waveguide near the end face on the external resonator side is not perpendicular to the end face. Cavity tunable laser.

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