JP4625661B2 - Semiconductor optical device, laser module, and optical transceiver - Google Patents

Description

  The present invention relates to a continuous wave laser, a wavelength tunable laser, a semiconductor light emitting device having a transmission speed of 2.5 Gbit / s or more, an optical amplifier, and a laser module equipped with them, and the technical field covers optical transceivers. .

  In general, when a semiconductor laser, an electroabsorption modulator, or a semiconductor optical amplifier is driven, the temperature of the element active layer rises due to local heat generation. When the temperature of the active layer portion changes, the characteristics of the semiconductor optical device change due to the change of the band gap energy of the semiconductor forming the active layer.

  Specifically, in the case of a semiconductor laser, when the temperature of the active layer increases, the oscillation threshold current increases, the current-light conversion efficiency (slope efficiency) decreases, the oscillation wavelength increases, and the like. In particular, the increase in the oscillation wavelength is about 0.1 nm / ° C. when the semiconductor material forming the active layer is an indium phosphide (InP) compound semiconductor in the 1.55 μm wavelength band.

  In the high-density wavelength division multiplexing (DWDM) optical communication that transmits optical signals having a plurality of channel wavelengths through a single optical transmission line, which is currently widely used in the trunk line system, the wavelength of each channel to be used. The interval is determined to be an extremely narrow interval of 0.4 nm or 0.8 nm.

  Therefore, if the oscillation wavelength fluctuates due to fluctuations in the active layer temperature of the laser in DWDM optical communication, optical signals of the respective wavelengths may affect each other, and it may be difficult to provide DWDM.

  In the case of a distributed feedback (DFB) laser (hereinafter referred to as an EA / DFB laser) in which an electro-absorption (EA) modulator is integrated, the temperature rise in the active layer of the laser part is applied to the modulator part. In addition to conducting, applying a reverse voltage to the modulator section also raises the temperature of the active layer of the modulator section.

  When the active layer temperature of the modulator part rises, the absorption edge wavelength of the modulator part increases. The increase amount is about 0.5 nm / ° C. in the case of a 1.55 μm wavelength band InP-based compound semiconductor. Therefore, in the case of the same wavelength band and the same semiconductor material, ΔH defined by the difference between the oscillation wavelength of the laser part and the absorption edge wavelength of the modulator part decreases by 0.4 nm per unit temperature. The decrease in ΔH accompanying the temperature rise in the active layer causes an increase in the extinction ratio, which is the ratio of light on / off, and a decrease in the α parameter that determines the chirping characteristics, thereby making the transmission characteristics of the EA / DFB laser constant. Can't keep up.

  On the other hand, for example, in Japanese Patent Application Laid-Open No. 2002-323865, means for controlling the temperature of the modulator unit independently of the laser unit by means of a thin film heater provided in the vicinity of the modulator unit to keep the element characteristics constant. It is shown.

  In the case of a semiconductor optical amplifier (SOA), the gain peak wavelength increases with an increase in temperature of the active layer, so that the amplification factor fluctuates and the output power after amplification is reduced.

  The laser modules incorporating these semiconductor optical elements include a type of cooled laser module that detects temperature fluctuations of the elements and controls the temperature of the semiconductor optical elements to keep the characteristics constant, and changes the driving conditions of the semiconductor optical elements. There is a type of uncooled laser module that keeps its characteristics constant. Examples of the cooled laser module include a module that requires wavelength control, such as a module for DWDM optical communication or a module equipped with a wavelength tunable laser. Moreover, as an uncooled laser module, there are many modules equipped with a direct modulation laser or an EA / DFB laser that requires lower cost and lower power consumption. In either case, the thermistor is disposed in the laser module, and the element temperature is detected by reading the resistance value.

  The thermistor is often placed on a chip carrier on which an element is mounted. The element temperature detected by the thermistor is fed back to the Peltier substrate immediately below the chip carrier in the case of a cooled laser module, and to the driving circuit of the semiconductor laser and modulator in the case of an uncooled laser module.

JP 2002-323865 A

  Most current laser modules detect the element temperature using the thermistor described above. However, the temperature detected by the thermistor is via the thermal resistance of the chip carrier or solder, and does not directly detect the temperature of the active layer portion of the element. Further, as long as the thermistor is used, it is difficult to reduce the size of the chip carrier or the laser module, and the mounting process also takes time.

  The problem to be solved by the present invention is to provide a small and low-cost laser module that directly detects the temperature of the active layer portion of an element without using a specific temperature detection component such as a thermistor.

  In order to solve the above-described problem, a region in which the temperature of the active layer portion can be measured is provided in the semiconductor optical device. Specifically, a region in which current-voltage (IV) characteristics can be measured is provided in the same semiconductor substrate, particularly in the vicinity of the element structure that generates the most heat. In this region, a mesa structure needs to be formed in a PIN junction sandwiching an intrinsic semiconductor that forms an active layer between a p-type semiconductor and an n-type semiconductor, or a PN junction composed of a p-type semiconductor and an n-type semiconductor. There is.

  When the mesa structure is a stripe shape, it is sufficient that the width and length are about several μm and 50 μm, respectively. In addition, when the mesa structure is circular, it is sufficient that its diameter is several μm. A p-electrode and an n-electrode are formed in such a semiconductor structure.

  The voltage value when a certain current is injected into the IV characteristic measurement region formed as described above is read. As for the current to be injected, especially when the IV characteristic measurement region is composed of a PIN junction, it is a very small DC current that does not cause laser oscillation in the IV characteristic measurement region, or the IV characteristic measurement. In order to suppress heat generation in the region as much as possible, a pulse current is desirable.

  Since the IV characteristic varies depending on the temperature of the active layer portion, the element temperature can be detected by the read voltage value. This utilizes the basic property of a semiconductor that the band gap energy decreases due to thermal expansion of the crystal lattice spacing when the temperature of the active layer portion increases.

  By using the present invention, it is possible to more directly detect the temperature of the active layer portion that greatly affects the characteristics of the semiconductor optical device, and to feed back accurate information to the temperature control mechanism and drive circuit of the laser module. Is possible. In addition, since temperature detection can be performed within the element, a temperature detection component such as a thermistor conventionally used is not required, and the cost of the laser module can be reduced.

  Furthermore, since it is possible to reduce the area for mounting the temperature detection component, not only does it lead to downsizing of the laser module, but also eliminates the need for mounting the temperature detection component. Reduction is possible.

  Specific examples relating to the present invention will be described in detail below.

  FIG. 1 is a perspective view of a semiconductor optical device according to a first embodiment of the present invention, which includes an EA / DFB laser (laser unit 101, electroabsorption modulator unit 102) and an IV characteristic measurement unit 103 (FIG. 1). 1 (a)) and a cross-sectional structure taken along the dotted line AA ′ (FIG. 1 (b)). The IV characteristic measuring unit 103 is desirably in the vicinity of the laser unit 101 that generates the most heat. A separation groove 104 is obtained by removing a contact layer formed of InGaAs or the like having a small resistance value in order to electrically separate the laser unit 101 and the modulator unit 102.

  FIG. 1B will be described in detail. Here, an Fe-doped InP buried structure using an n-type InP semiconductor substrate, which is a general structure as an EA / DFB laser, is shown. 105 is an active layer composed of n-type InP, 106 is InGaAsP, 107 is p-type InP, 108 is a contact layer, 109 is Fe-doped InP, 110 is a semiconductor protective film, 111 is a p-electrode, 112 is an n-electrode is there.

  In FIG. 1B, the IV characteristic measurement unit 103 has the same semiconductor multilayer structure as the laser unit 101, but may have a multilayer structure of the modulator unit 102 or a simple PN junction. In any case, when the IV characteristic measurement unit 103 is manufactured, compared to a conventional EA / DFB laser that does not have an IV characteristic measurement unit, the formation of a multilayer structure, the formation of a stripe pattern, and the formation of electrodes, Since it can be manufactured in the same process as those of the laser part and the modulator part, the manufacturing cost does not increase.

  FIG. 2 shows an active layer temperature of a voltage value when a constant pulse current of 65 mA is injected into the IV characteristic measuring unit 103 in which a mesa stripe structure having a length of 400 μm and a width of 1.3 μm is formed in the same multilayer structure as the laser part. Indicates dependency. When the temperature of the active layer portion increases, the band gap energy decreases and the voltage value decreases. The heat generated in the active layer part of the laser part 101 is conducted to the active layer part of the IV characteristic measuring part 103 through the surrounding semiconductor.

  Therefore, it is possible to directly detect the temperature of the active layer portion of the laser portion 101 by reading the voltage value when a constant current is injected into the IV characteristic measuring portion 103. By detecting the temperature of the active layer portion, for example, the temperature of the element can be controlled to stabilize ΔH, that is, stabilize the characteristics.

  FIG. 3A shows an element structure in which the direct modulation laser 301 includes an IV characteristic measurement unit 302. FIG. 3B is a cross-sectional structure taken along the dotted line BB ′ shown in FIG. Here, a ridge structure using an n-type InP substrate, which is a general structure as a direct modulation laser, is shown. Reference numeral 303 denotes an n-type InP, 304 an active layer formed of InAlAsP, 305 a p-type InP, 306 a contact layer, 307 a semiconductor protective film, 308 a p-electrode, and 309 an n-electrode.

  In this case as well, as in the case of the EA / DFB laser element of the first embodiment, it is desirable that the IV characteristic measurement unit 302 is disposed in the vicinity of the direct modulation type laser 301. However, in the ridge structure in which the laser unit 301 and the IV characteristic measurement unit 302 are connected by the same active layer, even when the laser beam oscillated by the laser unit 301 propagates to the IV characteristic measurement unit 302 The separation grooves 310 are required to be separated to such an extent that the strength becomes sufficiently weak, or deep enough to penetrate the active layer 304 as shown in FIG.

  The semiconductor multilayer structure of the IV characteristic measurement unit 302 may be a simple PN junction as in the first embodiment. In any case, when the IV characteristic measurement unit 302 is manufactured, a new manufacturing process does not increase.

  The heat generated in the active layer part of the laser part 301 is conducted to the active layer part of the IV characteristic measuring part 302 through the active layer and the surrounding semiconductor. In this embodiment, since the semiconductor material forming the active layer is different, the fluctuation amount due to the band gap energy and the temperature is different from that of the EA / DFB laser of the first embodiment. The temperature dependence on the part shows the same tendency as in FIG. Therefore, it goes without saying that even in the case of a direct modulation laser, the temperature of the active layer of the laser part can be directly detected from the voltage value when a certain constant current is injected.

  FIG. 4 shows a structure when an element provided with an IV characteristic measurement unit in the EA / DFB laser described in the first embodiment is mounted on a cooled laser module that requires temperature control of the element. Here, 404 is an EA / DFB laser (401: semiconductor laser unit, 402: electroabsorption modulator unit, 403: IV characteristic measurement unit), 405 is a termination resistor, 406 is a Peltier substrate, and 407 and 408 are respectively Front and rear output light from the EA / DFB laser, 409 is a condensing lens for fiber-coupling the front output light 407, and 410 is a monitor photodiode (hereinafter referred to as monitor PD) for receiving the rear output light 408. 411 is a package of a cooled laser module.

  In this laser module, APC (Auto Power Control) control is performed in which the fluctuation of the rear output light power received by the monitor PD 410 is fed back to the laser drive current source to keep the front output light power constant. Further, using the method described in the first embodiment, the temperature of the element active layer portion is detected from the voltage value measured by the IV characteristic measurement unit 503 via the voltage / temperature conversion unit. Then, this temperature is fed back to the Peltier substrate disposed immediately below the chip carrier on which the element is mounted (the chip carrier is omitted in FIG. 4), and the temperature of the element active layer portion is always kept constant. (Temperature Control) control is performed.

  In general, a laser module must satisfy certain characteristics under a wide ambient temperature environment of 0 ° C to 70 ° C. In the case of a laser module for DWDM optical communication, it is necessary to suppress wavelength fluctuation as much as possible regardless of how the surrounding environment changes. However, in the conventional laser module, when the ambient temperature fluctuates from 0 ° C. to 70 ° C., a wavelength variation of about ± 20 pm occurs.

  This is because the temperature of the semiconductor optical element detected by the thermistor disposed on the chip carrier is a temperature including variations due to the thermal resistance of the chip carrier, the non-uniformity of the solder, and the like. Therefore, if the ATC control method of the present embodiment is used, the element temperature can be directly detected, and the wavelength fluctuation due to the change in the surrounding temperature environment can be reduced from the conventional ± 20 pm.

  FIG. 5 shows a structure when an element provided with an IV characteristic measurement unit in the EA / DFB laser described in the first embodiment is mounted on an uncooled laser module that does not perform temperature control of the element. Here, 504 is an EA / DFB laser (501: semiconductor laser unit, 502: electroabsorption modulator unit, 503: IV characteristic measurement unit), 505 is a termination resistor, and 506 and 507 are EA / DFB lasers, respectively. 508 is a condensing lens for fiber-coupling the front output light 506, 509 is a monitor PD for receiving the rear output light 507, and 510 is an uncooled laser module package.

  As described above, if the temperature is not controlled in the EA / DFB laser, ΔH decreases as the temperature of the element active layer portion rises, causing an increase in the extinction ratio, for example. In order to keep the characteristics constant even when the temperature of the element active layer portion fluctuates, it is necessary to change the driving conditions of the electroabsorption modulator 502. Specifically, when the element temperature rises, the amplitude of the high-frequency signal applied to the modulator section may be reduced, or the on-level voltage value (bias voltage value) of the high-frequency signal may be increased.

  In this way, in the uncooled laser module, the element characteristics are fixed by feeding back the temperature of the element active layer detected by the IV characteristic measurement unit 503 to the bias / amplitude control unit of the high frequency signal applied to the modulator unit 502. Keep on.

  FIG. 6 shows a structure when an element provided with an IV characteristic measurement unit in the direct modulation laser described in the second embodiment is mounted on an uncooled laser module. Here, 603 is a direct modulation type laser (601: laser unit, 602: IV characteristic measurement unit), 604 and 605 are front and rear output lights from the direct modulation type laser 603, and 606 is a front output light 604. A condensing lens for fiber coupling, 607 is a monitor PD for receiving the backward output light 605, and 608 is an uncooled laser module package.

  As described above, in the direct modulation laser, the oscillation threshold current increases and the slope efficiency decreases as the temperature of the element active layer portion increases, causing a decrease in light output and an increase in extinction ratio. As in the fourth embodiment, it is necessary to change the driving conditions of the laser unit 601 in order to keep the characteristics constant when the temperature of the element active layer portion fluctuates. Specifically, the fluctuation of the rear output light power is detected by the monitor PD 607, and this is fed back to the bias control unit of the laser unit 601 to perform APC control. Further, the temperature variation of the element active layer portion is detected by the IV characteristic measurement unit 602, which is fed back to the modulation current control unit of the laser unit 601 to perform ATC control.

  In this way, it is possible to keep the characteristics of an uncooled laser module equipped with a direct modulation laser constant.

  FIG. 7 shows an IV characteristic measurement unit 704 in a semiconductor optical device in which a wavelength tunable DFB laser including a plurality of arrayed DFB laser units 701, a multimode interference multiplexer 702, and an SOA 703 are integrated. It is the structure which provided. In each of the plurality of arrayed DFB laser units 701, diffraction gratings having different pitches are formed.

  FIG. 8 shows a plurality of arrayed active region units 801 and distributed Bragg reflector (DBR) units 802, a tunable DBR laser, a multimode interference multiplexer 803, an SOA 804, This is a structure in which an IV characteristic measurement unit 805 is provided in a semiconductor optical device in which is integrated. Reference numeral 806 denotes a separation groove for electrically separating the active region portion 801 and the DBR portion 802. Each of the plurality of arrayed DBR portions 802 is formed with diffraction gratings having different pitches. As in the first and second embodiments, in FIGS. 7 and 8, the IV characteristic measurement units 704 and 805 are preferably in the vicinity of the laser unit that generates the most heat.

  7 and 8 are both wavelength tunable lasers, but the methods for changing the oscillation wavelength are different between the two. In the arrayed wavelength tunable DFB laser of FIG. 7, each DFB laser oscillates in a certain wavelength range by changing the temperature of the active layer portion. Since the diffraction grating pitch is different for each DFB laser, the oscillation wavelength can be varied over a wide range as the entire array.

  On the other hand, in the arrayed wavelength tunable DBR laser of FIG. 8, laser oscillation is caused by injecting current into the active region 801 having an optical amplification function, and the current value injected into the DBR portion is changed. The oscillation wavelength is controlled by changing the reflectance with respect to the wavelength. Since the diffraction grating pitch is different in each DBR section, the oscillation wavelength can be varied over a wide range as the entire array.

  In either case, the oscillation wavelength can be kept constant by directly detecting the temperature of the element active layer portion. Further, the SOA gain can be kept constant, which leads to stabilization of output power.

  FIG. 9 shows a structure when the tunable DFB laser shown in FIG. 7 is mounted on a cooled laser module. Here, 904 is a wavelength tunable laser (901: DFB laser (shows one of the arrayed lasers), 902: SOA, 903: IV characteristic measurement unit), 905 is a Peltier substrate, 906 is front output light, Reference numeral 907 denotes a beam splitter, reference numeral 908 denotes a monitor PD, reference numeral 909 denotes a condensing lens for fiber-coupling forward output light 906, and reference numeral 910 denotes a cooled laser module package.

  In many cases, the tunable laser module performs ACC (Auto Current Control) control to keep the current value injected into the laser unit constant. On the other hand, for APC control, the light output is kept constant by feeding back the fluctuation of the front output light received by the monitor PD 908 to the SOA drive current source. Further, the temperature of the element active layer portion is detected through the voltage value measured by the IV characteristic measurement unit 903 via the voltage / temperature conversion unit. The temperature is returned to the Peltier substrate to perform ATC control.

  Conventionally, a wavelength locker module comprising etalon and PD as main components has been used for ATC control. That is, the wavelength of the forward output light 906 is monitored by the wavelength locker module, the temperature fluctuation of the element active layer portion is read from the wavelength fluctuation amount, and is fed back to the Peltier substrate 905. However, since the ATC control can be performed without using the wavelength locker module by using the module of this embodiment, the size and cost of the module can be greatly reduced.

  By incorporating the laser modules described in Embodiments 3 to 5 and Embodiment 7 described above into an optical transceiver, it is possible to realize a compact and low-cost optical transceiver with stable characteristics.

  According to the present invention, it is possible to realize a small-sized and low-cost laser module and an optical transmitter / receiver having stable characteristics as compared with a conventional laser module, which is an industrially very effective invention.

It is a figure which shows the element structure and sectional structure which provided the IV characteristic measurement area | region in the electroabsorption type modulator integrated laser. It is a figure which shows the active layer part temperature dependence of a voltage value when a fixed electric current is inject | poured into an IV characteristic measurement part. It is a figure which shows the element structure and sectional structure which provided the IV characteristic measurement area | region in the direct modulation type | mold laser. It is a figure which shows the structure of the cooled laser module carrying an electroabsorption type modulator integrated laser. It is a figure which shows the structure of the uncooled laser module carrying an electroabsorption type modulator integrated laser. Structure of an uncooled laser module equipped with a direct modulation laser. It is a figure which shows the element structure which provided the IV characteristic measurement area | region in the wavelength tunable DFB laser. It is a figure which shows the element structure which provided the IV characteristic measurement area | region in the wavelength variable DBR laser. It is a figure which shows the structure of the cooled laser module carrying a wavelength tunable DFB laser.

Explanation of symbols

101, 301, 401, 501, 601 ... Semiconductor laser part, 102, 402, 502 ... Electroabsorption modulator part, 103, 302, 403, 503, 602, 704, 805, 903 ... I -V characteristic measurement unit, 104, 310, 806 ... separation groove, 105, 303 ... n-type InP, 106 ... InGaAsP active layer, 107, 305 ... p-type InP, 108, 306,. Contact layer, 109 ... Fe-doped InP, 110, 307 ... Semiconductor protective film, 111,308 ... p electrode, 112,309 ... n electrode, 304 ... InAlAsP active layer, 404, 504: Electroabsorption modulator integrated laser, 405, 505 ... Termination resistor, 406, 905 ... Peltier substrate, 407, 506, 604, 906 ... Previous Output light, 408, 507, 605 ... Back output light, 409, 508, 606, 909 ... Condensing lens, 410, 509, 607, 908 ... Monitor photodiode, 411, 910,. · Cooled laser module package, 510, 608 ... uncooled laser module package, 701, 901 ... DFB laser, 702, 803 ... Other mode interference multiplexer, 703, 804, 902 ... SOA , 801 ... Active region part, 802 ... DBR part, 904 ... Tunable DFB laser, 907 ... Beam splitter

Claims (2)

A laser module that does not perform temperature control by a temperature adjustment element for a semiconductor optical element composed of a laser part and a modulator part,
The semiconductor optical device has a temperature measurement region for measuring the temperature of the active layer,
The laser module includes a first drive circuit that drives the laser unit and a second drive circuit that drives the modulator unit, and an increase in the temperature of the active layer was observed using the temperature measurement region. In this case, the control is performed so as to change the drive condition of only the second drive circuit , and
The temperature measurement region is for measuring current-voltage characteristics,
When an increase in the temperature of the active layer is observed using the temperature measurement region, control is performed to reduce the amplitude of the high frequency signal applied to the modulator section or increase the bias voltage value of the high frequency signal. A laser module characterized by that.   An optical transceiver comprising the laser module according to claim 1 mounted thereon.

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