JP5171538B2 - Optical modulator and tunable laser module - Google Patents

Description

  The present invention relates to an optical modulator that modulates the intensity of laser light and a wavelength tunable laser module.

  In a long-distance optical communication system, waveform deterioration of an optical signal due to wavelength dispersion of an optical fiber that is a transmission medium affects the length of the relay distance and greatly affects the system cost. In particular, in a high-capacity high-speed communication system, this influence is serious, and an external modulation method with less fluctuation of the wavelength included in the optical signal, so-called chirping, is employed to suppress waveform deterioration. The devices that can be used for the external modulation system are as follows: 1) an absorption semiconductor optical modulator having a multiple quantum well (MQW) structure that utilizes an increase in light absorption by applying an electric field, and 2) a Mach-Zehnder interference system that utilizes an electro-optic effect. LN modulators have been put into practical use.

  On the other hand, a wavelength division multiplexing (WDM) communication system has been introduced to further increase the capacity of an optical communication system. As a light source of the WDM communication system, a wavelength tunable laser capable of controlling and changing the wavelength of laser light is suitable. As a result of technological development in recent years, practical application of wavelength tunable lasers has become a reality, and introduction into practical systems is about to begin.

  Among the above-described external modulators, the absorption semiconductor optical modulator is limited in the wavelength band that can be modulated due to its operating principle, and therefore should be applied as a modulator for dealing with such a wavelength tunable light source. Is impossible. Therefore, a Mach-Zehnder interferometric LN modulator is considered promising, but this device has a problem that it is too large to be mounted on a small transceiver module.

  Patent Document 1 discloses an optical modulator with less chirp using a change in refractive index due to an electro-optic effect. Unlike the conventional absorption-type semiconductor optical modulator that modulates light absorption by controlling the optical absorption with an electric field, the operable wavelength band is not limited in this optical modulator. Therefore, it can be used as a modulator of a wavelength tunable laser and can be miniaturized. Patent Document 2 discloses a Mach-Zehnder interference type optical modulator using silicon as a material. This device has attracted attention as an optical modulator made of silicon, which can reduce the manufacturing cost.

JP 2003-233040 A WO2004 / 088394

  However, the optical modulator disclosed in Patent Document 1 is a modulator with less chirp than a conventional absorption semiconductor optical modulator, but is used for a long-distance high-speed communication system of 10 Gbps or more. Is insufficient. Moreover, since the optical modulator described in Patent Document 2 is a Mach-Zehnder interference type, it does not solve the problem of a large device size.

  In view of the above problems, an object of the present invention is to realize a small-sized optical modulator that has a wide operable wavelength band and that does not undergo waveform deterioration even with respect to dispersion by an optical fiber.

In order to solve the above problems, an optical modulator according to the present invention is an optical modulator that superimposes a signal on laser light, and an optical waveguide layer for guiding laser light and an optical waveguide layer interposed therebetween. A first clad layer and a second clad layer which are provided between the optical waveguide layers and have a refractive index smaller than that of the optical waveguide layer; and two reflectors which are provided to face each other in order to multiply reflect the laser light propagating through the optical waveguide layer, , is disposed between two reflectors, comprising an electrode modulation signal for changing the refractive index of the optical waveguide layer is applied, the electrode, the refractive index of said optical waveguide layer by modulation signal follow the relatively higher, and wherein said arc opposed to the intensity of the two signal light output to the outside of the reflector provided is biased so that a high.

  The optical modulator according to the present invention has a Fabry-Perot interferometer structure in which reflectors are provided at both ends of an optical waveguide to multiplexly reflect laser light. When operating this optical modulator by adding a modulation signal, the intensity of the modulated laser beam output to the outside increases as the modulation signal changes in the direction of increasing the refractive index of the waveguide. Set conditions. As a result, it is possible to obtain modulated light with controlled chirp characteristics in which signal waveform deterioration is reduced after transmission through an optical fiber.

  In the optical modulator according to the present invention, the two reflectors are constituted by a cleaved surface formed at the end of the optical waveguide layer or a mirror surface on which a coating for adjusting the reflectance is applied to the cleaved surface. It is characterized by being. Thereby, the size of the optical modulator can be reduced, and the configuration of the optical system can be simplified.

  In the optical modulator according to the present invention, the two reflectors are Bragg reflectors having a diffraction grating that interferes with laser light propagating through the optical waveguide layer. Furthermore, the optical modulator according to the present invention has an antireflection film for preventing reflection of laser light on the cleaved surface provided on both end faces of the optical waveguide layer.

  In the optical modulator according to the present invention, the first cladding layer is made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system containing InP that is transparent to laser light propagating through the optical waveguide layer, and the second cladding layer Is made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system containing InP, and the optical waveguide layer includes a compound semiconductor layer selected from an InGaAsP mixed crystal system.

  Furthermore, in the optical modulator according to the present invention, the optical waveguide layer includes a barrier layer made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system containing InP, and a well made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system. And a multiple quantum well (MQW) structure in which layers are stacked.

  In the optical modulator according to the present invention, the optical waveguide layer includes a barrier layer made of a compound semiconductor crystal selected from an AlGaInAs mixed crystal system that is transparent to laser light propagating through the optical waveguide layer, and an InGaAsP mixed crystal. It has a multiple quantum well (MQW) structure in which a well layer made of a compound semiconductor crystal selected from the system is stacked.

  The above-described InGaAsP mixed crystal system compound and AlGaInAs mixed crystal system compound semiconductor, which are materials for the optical modulator of the present invention, are mixed crystal systems that lattice match with InP. Therefore, a commercially available InP substrate can be formed by a crystal growth method such as MOCVD. A high-quality crystal with few crystal defects such as dislocations can be formed thereon. In addition, it has a crystal composition that is transparent to a wavelength band of 1.5 μm used for optical fiber communication. Therefore, it is suitable as a material for the optical modulator of the present invention.

  In the optical modulator according to the present invention, the first cladding layer has a first conductivity, the second cladding layer has a second conductivity, and each of the first cladding layer and the second cladding layer has a second conductivity. The refractive index of the optical waveguide layer is changed by applying a modulation signal having a reverse bias to the two electrodes connected to.

  According to the present invention, a modulation signal having a reverse bias is applied to the PN junction located between the first cladding layer and the second cladding layer, and a modulated electric field is applied to the optical waveguide layer. As a result, the refractive index changes due to the electro-optic effect, the boundary conditions of the Fabry-Perot interferometer change, and the intensity of the output light changes. As a result, the output light is subjected to intensity modulation corresponding to the modulation signal, and an operation as an optical modulator is obtained.

  In the optical modulator according to the present invention, the first cladding layer and the second cladding layer are made of a dielectric layer that is transparent to the laser light propagating through the optical waveguide layer, and the optical waveguide layer includes the first cladding layer. Provided between the first and second cladding layers and having a configuration in which a P-type silicon layer, a dielectric layer, and an N-type silicon layer are stacked, and two electrodes for applying a modulation signal are P-type silicon It is characterized by being connected to each of the layer and the N-type silicon layer.

In the optical modulator according to the present invention, when a modulation signal is applied to two electrodes, holes and electrons at the interface between the dielectric film provided on the optical waveguide layer and the P-type silicon layer and the N-type silicon layer Concentration changes. That is, when N-type silicon is grounded to an earth potential and a positive voltage is applied to P-type silicon, holes are accumulated at the interface on the P-type silicon side, and electrons are accumulated at the interface on the N-type silicon side. As a result, the carrier concentration in the vicinity of the dielectric film increases. On the other hand, when a negative voltage is applied to the P-type silicon, a depletion region in which carriers are discharged is formed at both the P-type and N-type interfaces. By applying a modulation signal, accumulation and discharge of carriers alternately occur at the interface of the dielectric film, and the refractive index of the optical waveguide layer changes corresponding to the change in carrier concentration. As a result, the boundary conditions of the Fabry-Perot interferometer change, the intensity of the output light is modulated, and the optical modulator operates. As a material used for the dielectric layer, for example, a silicon oxide film (SiO ₂ ) can be used.

  Furthermore, in the optical modulator according to the present invention, the two reflectors are configured such that a free spectral range (FSR), which is an interval between adjacent resonance frequencies of the opposing reflectors, matches a predetermined interval. Features.

  In the Fabry-Perot interferometer-type optical modulator, the transmittance of light having a frequency that matches the resonance frequency of the reflectors arranged opposite to each other is maximized. As a result, the optical modulator according to the present invention has a property of transmitting only light having a predetermined frequency separated by an FSR that is an interval between adjacent resonance frequencies. On the other hand, in a WDM optical communication system, a plurality of laser light sources having a constant frequency interval are used. Therefore, an optical modulator capable of modulating the light of all the laser light sources used in the system by matching the FSR of the optical modulator with the frequency interval of the laser light source used for WDM optical communication is realized. Can do. For example, an extremely useful optical modulator can be realized if the FSR is matched with an ITU grid interval (International Telecommunication Union grid spacing) which is an international standard.

  In addition, a wavelength tunable laser module according to the present invention includes a wavelength tunable laser capable of changing the wavelength of output light, and the optical modulator described above. Furthermore, the wavelength tunable laser module according to the present invention includes an optical isolator for attenuating light returning to the wavelength tunable laser between the wavelength tunable laser and the optical modulator.

  The laser module of the present invention can output modulated light having a plurality of wavelengths by including the optical modulator and the wavelength tunable laser. Thereby, a useful laser module with a high degree of freedom when used as a light source of a WDM optical communication system can be realized. Also, by arranging an optical isolator between the wavelength tunable laser and the optical modulator, it is possible to block the return light reflected by the reflection mirror of the optical modulator and returning to the wavelength tunable laser. Thereby, the stable operation | movement of a wavelength variable laser is securable.

  According to the optical modulator of the present invention, it is possible to realize a small-sized optical modulator that has a wide operable wavelength band and does not undergo waveform deterioration even with respect to dispersion caused by an optical fiber.

DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram schematically showing an optical modulator 1 according to the present invention. An optical waveguide layer 4 for guiding laser light input from one end face to the other end face and outputting it to the outside; an N-type clad layer 2 as a first clad layer sandwiching the optical waveguide layer 4; And a P-type cladding layer 3 as a second cladding layer. The optical waveguide layer 4 is a conductive semiconductor layer, and a transparent material that does not absorb light with respect to the input laser beam is used. The N-type cladding layer 2 and the P-type cladding layer 3 are semiconductor layers having N-type and P-type conductivity, respectively, and a material that is transparent to laser light is used. Further, the N-type cladding layer 2 and the P-type cladding layer 3 are made of a material having a refractive index smaller than that of the optical waveguide layer 4 in order to confine the laser beam in the optical waveguide layer 4 and guide it.

  The optical waveguide layer 4 may have either P-type or N-type conductivity. As will be described later, when the optical modulator 1 is operated by applying a reverse bias to the PN junction located at the interface between the N-type cladding layer 2 or the P-type cladding layer 3 and the optical waveguide layer 4, A configuration in which a high electric field is applied to the whole is preferable. For this purpose, it is necessary to lower the carrier concentration of the optical waveguide layer 4, and it is preferable to use a PIN structure diode as a so-called I layer having a high resistance.

  Reflecting mirrors 5 set to a predetermined reflectance are provided on both end faces of the optical modulator 1. The laser light guided along the optical waveguide layer 4 is multiple-reflected between a pair of reflecting mirrors 5 provided opposite to both end faces of the optical modulator 1, and then output to the outside. A P electrode 6 and an N electrode 7 for applying a modulation signal to the optical modulator 1 are provided between the pair of reflection mirrors 5. The P electrode 6 is ohmically connected to the P cladding layer 3, and the N electrode 7 is ohmically connected to the N cladding layer 2.

ここで、Ｉ_０は入力光の強度であり、Ｔ１、Ｔ２は、それぞれ入力側、出力側の反射ミラー５の透過率、Ｒ１、Ｒ２は、それぞれ入力側、出力側の反射ミラー５の反射率を示している。また、ｎは、光導波層４の屈折率であり、Ｌは対向する反射ミラー５の間隔、所謂共振器長である。λ_０は、レーザー光の真空中の波長である。 The light intensity I output from the light modulator 1 to the outside by multiple reflection between the pair of reflecting mirrors 5 facing the laser light input to the light modulator 1 is expressed by the following equation.

Here, I ₀ is the intensity of the input light, T1 and T2 are the transmittances of the reflection mirror 5 on the input side and the output side, respectively, and R1 and R2 are the reflectances of the reflection mirror 5 on the input side and the output side, respectively. Is shown. Further, n is the refractive index of the optical waveguide layer 4, and L is the distance between the opposing reflecting mirrors 5, so-called resonator length. λ ₀ is the wavelength of the laser light in vacuum.

2 and 3 show the relative intensity (| I | / | I ₀ |) of the output light of the optical modulator 1 with respect to the input light, that is, the light transmission characteristics of the modulator 1, and the optical frequency (c ₀ / λ ₀ : c ₀ is a schematic diagram shown with respect to light speed in a vacuum). The transmittance of the optical modulator is maximized when an integral multiple of a half wavelength λ / 2 (= λ ₀ / 2n) of light in the optical waveguide 4 is equal to L. The optical frequency when the integral multiple of λ / 2 is equal to L is the above-described resonance frequency, and the interval between the peaks of the modulator transmittance shown in the figure is FSR.

  In the light transmission characteristic 10 of the light modulator indicated by a solid line in FIG. 2, the transmittance of the light modulator 1 with respect to the input light shown in the figure is the smallest. When a reverse bias is applied to the optical modulator 1, the refractive index of the optical waveguide layer 4 increases due to the electrooptic effect, and the light transmission characteristic 10 shifts in the direction of the arrow in the figure. At this time, when shifting to the light transmission characteristic 11 shown by a broken line in the figure, the frequency of the input light coincides with the resonance frequency, and the transmittance for the input light is maximized. That is, the output light can be modulated from OFF to ON by applying a reverse bias having a magnitude that shifts from the bias point indicating the light transmission characteristic 10 to the light transmission characteristic 11. On the other hand, when the reverse bias is lowered and returned to the original bias point, it is clear that the output light is modulated from ON to OFF.

  In FIG. 3, unlike the case shown in FIG. 2, the frequency of the input light coincides with the resonance frequency at the bias point of the light transmission characteristic 10, and the transmittance with respect to the input light is maximized. When a reverse bias is applied to the optical modulator 1 and the refractive index of the optical waveguide layer 4 increases, the light transmission characteristic 10 shifts in the direction of the arrow in the figure. When shifting to the light transmission characteristic 11 shown by the broken line, the transmittance with respect to the input light is minimized, and the output light is modulated from ON to OFF. That is, unlike the case shown in FIG. 2, by applying a reverse bias to the optical modulator 1, the output light is modulated from ON to OFF. In the following description, the case shown in FIG. 2 that is modulated from OFF to ON when a reverse bias is applied is assumed to be the normal phase, and the case where the modulation is applied from ON to OFF shown in FIG.

  A conventional absorption semiconductor optical modulator operates in a reverse-phase mode in principle. Also in the optical modulator described in Patent Document 1, the operation is in the reverse phase mode. That is, the semiconductor optical modulator is known to operate in the reverse phase mode and has become common technical knowledge. However, it has been found that the optical modulator 1 according to the present invention exhibits remarkable operating characteristics not found in conventional optical modulators when operated in the normal phase mode.

  FIG. 4 shows the intensity ratio (relative light intensity) of the light output from the optical modulator 1 to the input light with respect to the reverse bias when operated in the normal phase mode. The intensity ratio is minimum at 0 bias, and the intensity ratio is maximum when −2 V is applied. That is, in the optical modulator 1, it can be seen that an extinction ratio of about 14 dB can be obtained by applying a modulation signal that gives a reverse bias with an amplitude of 2V. Here, the reflectance R (= R1 = R2) of the reflecting mirror of the optical modulator 1 is 0.75, the transmittance T (= T1 = T2) is 0.25, and the resonator length L is 150 μm.

ここで、Φは、レーザー光の位相であり、ｎは光導波層の屈折率である。上式から分かるように、周波数変動Δωは屈折率の変化に逆比例する。 As described above, when a pulse signal having an amplitude of −2 V is applied as the modulation signal, the optical modulator 1 shown in FIG. 4 outputs an optical pulse having an extinction ratio of about 14 dB. The upper part of FIG. 5 shows a waveform of light output from the optical modulator 1 when a laser beam having a constant intensity is input and subjected to pulse modulation by a modulation signal applied to the electrode. On the other hand, when the laser light is modulated into an optical pulse in this way, if the refractive index changes in the optical waveguide layer 4, the frequency of the laser light varies. This is a so-called chirping phenomenon. The frequency variation Δω due to chirping is expressed by the following equation.

Here, Φ is the phase of the laser beam, and n is the refractive index of the optical waveguide layer. As can be seen from the above equation, the frequency variation Δω is inversely proportional to the change in refractive index.

  The lower part of FIG. 5 shows the frequency fluctuation Δω corresponding to the optical pulse waveform shown in the upper part. As described above, the optical modulator 1 in FIG. 5 operates in the normal phase mode. That is, at the rise of the optical pulse, the pulse of the modulation signal rises so that a voltage of −2 V is applied, and the refractive index of the optical waveguide layer 4 changes in the increasing direction. At this time, as shown in the lower part of FIG. 5, the frequency fluctuation Δω is on the negative side. On the other hand, at the trailing edge of the optical pulse, the modulation signal pulse also falls in the direction of 0, and the refractive index of the optical waveguide layer 4 decreases. Therefore, the frequency variation Δω varies to the plus side.

  FIG. 6 shows the relationship between the optical pulse waveform and the frequency fluctuation when the optical modulator 1 is operated in the reverse phase mode. In this case, the reverse bias modulation pulse rises at the fall of the optical pulse, and the refractive index of the optical waveguide layer 4 increases. Therefore, as shown in the figure, the frequency fluctuation Δω is on the negative side at the falling edge of the optical pulse, and is on the positive side at the rising edge of the optical pulse.

  When the frequency variation Δω is negative, it means that the wavelength of the light included in the optical pulse is long, and when it is positive, it means that the light wavelength is short. Therefore, when operating in the normal phase mode as shown in FIG. 5, light having a relatively long wavelength is included in the rising portion of the optical pulse, and light having a short wavelength is included in the falling portion. . On the other hand, in the operation in the anti-phase mode shown in FIG. 6, light having a relatively short wavelength is included at the rising edge of the optical pulse, and light having a long wavelength is included at the falling edge.

  FIG. 7 schematically shows a change in pulse waveform that occurs when the above-described optical pulse is transmitted through an optical fiber. FIG. 7A shows an optical pulse waveform after modulation, and FIG. 7B conceptually shows a waveform after an optical pulse modulated in the normal phase mode is transmitted through an optical fiber. FIG. 7D shows a waveform after the optical pulse modulated in the reverse phase mode is transmitted through the optical fiber. FIG. 7C shows a pulse waveform after transmitting an optical pulse that does not include light having different wavelengths at the rising and falling portions as a comparative example when chirping does not occur. is there.

  The optical fiber has chromatic dispersion in a wavelength band of 1.5 μm used for optical communication, and has a property that the transmission speed becomes slower as the optical wavelength is longer. For this reason, light having a long wavelength at the rising portion of the light pulse and modulated in a normal phase mode including a light component having a short wavelength at the falling portion is relatively light with a long wavelength at the rising portion. The light waveform is included in the later portion with a delay, and the light having a short wavelength in the falling portion catches up with the previous portion and is included. As a result, as shown in FIGS. 7B and 7C, the optical pulse modulated in the normal phase mode has a narrower pulse width after transmission than the waveform after transmission of the optical pulse without chirping. On the other hand, in the case of an optical pulse modulated in the anti-phase mode, light having a short wavelength contained in the rising portion proceeds first, and light having a long wavelength contained in the falling portion is delayed, and FIG. ), The pulse width becomes wide.

  The difference in pulse width after the above-described optical fiber transmission, that is, the degree of deterioration of the signal waveform is a big problem that affects transmission characteristics in a high-speed optical communication system. In high-speed optical communication, since the bit rate of optical signals is high, the interval between optical pulses is narrow. Therefore, if the signal waveform is deteriorated and the width of the optical pulse is widened, the optical pulses interfere with each other and the signal cannot be identified.

  FIG. 8 shows an eye pattern with 10 Gbps signal light modulated by the optical modulator 1. 9 and 10 show eye patterns after 10 Gbps signal light is transmitted through an optical fiber. The optical fiber of the transmission line is a normal single mode fiber having a wavelength dispersion of 80 km in length and 1600 ps / nm.

  FIG. 9 shows an eye pattern when an optical pulse modulated in the normal phase mode is transmitted. It can be seen that a clear eye opening is observed when the relative light intensity range shown on the vertical axis is between 0.1 and 0.9, and an error-free identification point can be set. On the other hand, FIG. 10 shows an eye pattern after transmission of an optical pulse modulated in the reverse phase mode. The lower half of the eye pattern is completely crushed, and the eye opening cannot be recognized.

  As can be seen by comparing the eye patterns in FIGS. 9 and 10, when the optical modulator 1 is modulated in the normal phase mode, an optical signal of 10 Gbps can be transmitted to a point 80 km away. In the case of modulation with, there is a remarkable difference that transmission is not possible at all. As described above, by setting the operating condition of the optical modulator 1 to the normal phase mode, deterioration of the optical pulse waveform can be prevented by using chirping. Thereby, it is possible to transmit the optical signal over a long distance.

  As a method for setting the operation mode of the optical modulator 1, a method is conceivable in which an offset voltage is applied so that the light transmittance is minimized, and then the modulation signal is transferred to the reverse bias. Further, since the refractive index of the optical waveguide layer can be changed by changing the temperature, the temperature during operation of the optical modulator 1 is controlled by an electronic temperature control device such as a Peltier cooler device, and the light transmittance at zero bias. It is conceivable to set the value to a minimum, and to combine with the application of an offset voltage.

  The optical modulator according to the present invention is not limited to an optical modulator that changes a refractive index by applying a reverse bias modulation signal to the PN junction, but the refractive index of the optical waveguide layer is determined by the modulation signal applied to the electrode. Modulator that can set operating conditions so as to increase the intensity of the signal light output to the outside of the two reflectors provided opposite to each other when the refractive index is relatively high Is included.

  FIG. 11 is a perspective view schematically showing the optical modulator 20 according to the first embodiment of the present invention. The optical modulator 20 is made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system containing InP epitaxially grown on the InP substrate 21 by MOCVD.

  On the N-type InP substrate 21 which is the first conductivity, the N-type InP layer 22 which is the first cladding layer, the N-type InGaAsP layer 23 which is the optical waveguide layer, and the P-type InP layer which is the second cladding layer 24, contact layer 28 is grown sequentially. The N-type InGaAsP layer 23 is selected to have a composition that lattice matches with InP and has an absorption edge wavelength of 1.3 μm (the InGaAsP mixed crystal described below is a crystal that lattice matches with InP). The contact layer 28 is a layer formed to obtain an ohmic junction with the P electrode 25 and is a P-type InGaAsP layer having an absorption edge wavelength of 1.1 μm.

The InP substrate 21 is a conductive substrate having a carrier concentration of 2 to ³ × 10 ¹⁸ cm ⁻³ . The N-type InP layer 22 has a thickness of about 3 μm and a carrier concentration of 2 to ³ × 10 ¹⁷ cm ⁻³ . The optical waveguide layer 23 has a thickness of 0.2 μm and a carrier concentration of 2 to ³ × 10 ¹⁵ cm ⁻³ . The optical waveguide layer 23 is preferably an I-type layer having a carrier concentration of 1 × 10 ¹⁴ cm ⁻³ or less, but is difficult to form by crystal growth by MOCVD. However, since the thickness is as thin as 0.2 μm, even the above carrier concentration can be operated with sufficient modulation characteristics. The P-type InP layer 24 that is the second cladding layer has a thickness of 2 μm and a carrier concentration of ³ to 5 × 10 ¹⁷ cm ⁻³ . The thickness of the second cladding layer is set to a thickness at which the laser light propagating through the optical waveguide layer 23 is not affected by absorption or reflection by the P electrode 25. It is determined in consideration of the optical confinement coefficient determined by the refractive index of the optical waveguide layer 23 and the refractive indexes of the InP layers 22 and 24 as the cladding layers. The contact layer 28 has a thickness of 0.2 μm and a carrier concentration of 1 to 2 × 10 ¹⁸ cm ⁻³ .

  The InP / InGaAsP (λ = 1.3 μm) / InP / InGaAsP (λ = 1.1 μm) heteroepitaxial layer grown on the InP substrate 21 is formed into a mesa stripe having a width of about 2 μm in order to form an optical waveguide. Etch. Etching is performed using a normal wet etching method with the silicon oxide film as a mask. Further, a normal plasma dry etching method using a chlorine-based reactive gas may be used. Further, the side surface of the mesa stripe is selectively buried by regrowth of the high resistance InP layer 29 to complete the optical waveguide. As the selective growth mask, a silicon oxide film used as a wet etching mask is used.

  In the above epitaxial growth, silicon (Si) is used as the N-type dopant. The P-type dopant uses zinc (Zn), and the high resistance InP layer 29 uses iron (Fe) that functions as a carrier killer. The first and second cladding layers may be InGaAsP layers instead of the InP layers 22 and 24 described above. However, after determining the optical confinement factor of the optical waveguide 23 in consideration of making the refractive index of the optical waveguide layer 23 higher than the refractive index of the clad layer, and the influence of the P electrode, the coupling with the input light, etc. Needless to say, the composition of the InGaAsP layer is determined.

After the optical waveguide is completed by the regrowth of the high resistance InP layer 29, the surface protective film 27 is formed on the surface of the epitaxial layer. The surface protective film 27 is made of a silicon oxide film (SiO ₂ ) and has a function of preventing migration of metal used for the P electrode 25 to the semiconductor crystal layer. Further, a contact hole is formed in the surface protective film 27 on the top surface of the mesa stripe that becomes an optical waveguide, and a P electrode 25 is formed in connection with the contact layer 28. The P electrode 25 uses a three-layer structure of titanium (Ti) / platinum (Pt) / gold (Au) in which metals are stacked. The contact layer 28 is in contact with the Ti layer to obtain an ohmic connection. The Au layer is provided for bonding a gold wire. The Pt layer is inserted to prevent diffusion between metals.

  The P electrode 25 has a portion provided along the mesa stripe for applying a modulation signal to the optical waveguide layer 23 and a bonding pad 25b provided for bonding a gold wire. The bonding pad 25b is provided on the high resistance InP layer 29 in order to reduce the stray capacitance. Thereby, it is possible to apply a high-speed modulation signal. On the other hand, an N electrode 26 is formed on the back surface of the InP substrate 21. For the N electrode 26, an alloy of gold (Au) and germanium (Ge) is used. An unnecessary portion of the InP substrate 21 is ground at the stage of the wafer process, and the thickness is adjusted to about 100 μm. This is because the modulator 20 can be easily cut into chips. The N electrode 26 is vacuum-deposited on the back surface after grinding.

  Furthermore, at the stage where the chip of the modulator 20 is cut out from the wafer, the chip is cut out so that the length in the mesa stripe direction becomes a predetermined resonator length L. Since the resonance frequency interval FSR described above is determined by the length of the resonator length L, it is necessary to process L with high accuracy. However, as will be described later, the operating temperature of the modulator 20 is controlled to adjust the FSR. Therefore, it is possible to manufacture with a processing accuracy of a resonator such as a normal semiconductor laser. In this embodiment, the resonator length L is 150 μm. In addition, a coating for adjusting the reflectance is applied to the chip end face.

FIG. 12 is a schematic diagram illustrating an AA cross section (see FIG. 11) of the modulator 20. An N-type InP layer 22 and an optical waveguide layer 23 that are first cladding layers, a P-type InP layer that is a second cladding layer, and a contact layer 28 are formed on an InP substrate 21 to form an optical waveguide. A P electrode 25 is formed on the contact layer 28. The P electrode 25 is in contact with the contact layer 28 in almost the entire region except for both ends of the optical waveguide, and applies a modulation signal to be reverse biased to the optical waveguide layer 23. As described above, since all of the InP substrate 21, the N-type InP layer 22, the P-type InP layer 24, and the contact layer 28 are doped at a high concentration, the interface between the P-type InP layer 24 and the optical waveguide layer 23 is present. Most of the reverse bias applied to the located PN junction is applied to the optical waveguide layer 23. As a result, even when a low voltage of about 2 V is applied, a high electric field of about 1 × 10 ⁵ V / cm is applied to the optical waveguide layer 23, resulting in an electro-optic effect.

Both end surfaces of the optical modulator 20 that are end surfaces of the optical waveguide are surfaces cut out from the wafer by cleaving and function as reflection mirrors. In the case of InP and InGaAsP mixed crystal, the reflectivity of the uncut cleavable surface without coating is about 60%. Therefore, even if the reflectance is not adjusted by applying a coating, it functions as two opposing reflectors of the light modulator. However, in order to optimize the modulation characteristics of the optical modulator, it is necessary to adjust to a predetermined reflectance. In this embodiment, the coating film 30 is formed on both end faces of the optical modulator 20 and the reflectance is adjusted to 75%. The coating film 30 is a multilayer film of a silicon oxide film (SiO ₂ ) and amorphous silicon (α-Si). The thickness of each layer is λ / 4 (λ is the wavelength of the laser beam) and is formed by plasma CVD.

  In the above embodiment, the optical waveguide layer 23 is a single layer of an InGaAsP layer (absorption edge wavelength 1.3 μm), but may be a multilayer quantum well (MQW) structure. For example, a well layer composed of an InGaAsP layer (absorption edge wavelength 1.1 μm) having a thickness of 10 nm and an InGaAsP layer (absorption edge wavelength 1.3 μm) having a thickness of 5 nm is alternately laminated on the N-type InP layer 22. An optical waveguide layer having a multiple quantum well structure is grown. The well layer has three layers and is sandwiched between four barrier layers. Further, a P-type InP layer 24 is laminated, and an optical modulator having an MQW-structured optical waveguide layer 23 can be manufactured by the same process as in the above embodiment.

  In the case of the optical waveguide layer 23 composed of a single layer of InGaAsP layer (absorption edge wavelength 1.3 μm), the refractive index is changed by an electro-optic effect called Franz-Keldish effect. In this case, the electro-optic effect called the Stark effect is the main component. Assuming that the electric field applied to the optical waveguide layer is the same, the Stark effect is larger in the change in the refractive index. Therefore, the modulation signal can be set to a low voltage by using the MQW structure for the optical waveguide layer. As the bit rate of the optical communication system increases, the IC output of the modulation circuit that generates the modulation signal tends to decrease. Therefore, it is effective to use an optical waveguide layer with an MQW structure and to operate with a low modulation signal. It is.

  As a material constituting the MQW structure, an AlGaInAs-based mixed crystal can be used. Specifically, an AlGaInAs layer (absorption edge wavelength: 1.0 μm) is used as the barrier layer. The thickness of the barrier layer is 10 nm as in the above case, and an MQW structure can be formed by alternately stacking well layers of InGaAsP layers (absorption edge wavelength 1.3 μm).

  The MQW structure that can be used for the optical waveguide layer is not limited to the above-described embodiments, and is realized by combining crystals of an InGaAsP mixed crystal system including InP and an AlInGaAs mixed crystal of an arbitrary composition. can do. Also, there is no requirement for special conditions for crystal growth such as MOCVD, and it is possible to use a standard growth apparatus used for manufacturing compound semiconductor devices such as semiconductor lasers.

  FIG. 13 is a schematic diagram showing an optical modulator 40 according to the second embodiment of the present invention. The optical modulator 40 is the same as the optical modulator 20 shown in Example 1 in that an InGaAsP mixed crystal compound semiconductor formed on the InP substrate 21 by MOCVD is used as a material, and is shown in FIG. Share the configuration. However, the difference is that the reflectors provided at both ends of the optical modulator are Bragg reflectors. FIGS. 13A, 13B, and 13C are diagrams schematically illustrating a manufacturing process of the Bragg reflector.

  FIG. 13A shows a state in which the diffraction grating 31 is formed on the N-type InP layer 22 formed on the InP substrate 21. In this embodiment, a 0.2 μm-thick InGaAsP layer (absorption edge wavelength: 1.1 μm) that becomes the diffraction grating 31 is formed on the N-type InP layer 22, and then the resist pattern of the diffraction grating is formed by electron beam exposure. Form. Further, using the resist pattern as a mask, the InGaAsP layer is etched by a dry etching method using a chlorine-based reaction gas to form the diffraction grating 31. The diffraction grating 31 is composed of thin InGaAsP arranged with a half-wavelength period of laser light propagating through the optical waveguide layer 23.

  FIG. 13B shows a state in which all of the compound semiconductor crystal layer of the optical modulator 40 has been grown. An N-type InP layer 32 is further formed on the N-type InP layer 22, and the diffraction grating 31 is embedded in the N-type InP layer. On the N-type InP layer 32, an optical waveguide layer 23, a P-type InP layer, and a contact layer 28 are formed.

FIG. 13C schematically shows an AA cross section of the completed optical modulator 40. A surface protective film 27 and a P electrode 25 are formed on the contact layer 28, and an N electrode 26 is formed on the back surface of the InP substrate. The diffraction grating 31 is located at both ends of the optical waveguide layer 23 and constitutes a pair of Bragg reflectors. The Bragg reflector reflects the laser light propagating through the optical waveguide layer 23 due to a periodic refractive index change caused by the diffraction grating 31 embedded in the N-type InP layers 22 and 32. Further, antireflection films 33 are provided on both end faces of the modulator to prevent the Bragg reflector and the reflecting mirror on the open surface from acting on the laser beam in an overlapping manner. The antireflection film 33 is provided by coating a dielectric film having a thickness of λ / 4 (λ is the wavelength of laser light). As the dielectric film, a silicon nitride film (Si ₃ N ₄ ) formed by a plasma CVD method can be used.

  The optical modulator 40 propagates through the optical waveguide layer 23 by applying a reverse-biased modulation signal between the P electrode 27 and the N electrode 26 in the same manner as the optical modulator 20 shown in the first embodiment. Modulates and outputs laser light. As described above, the optical waveguide layer 23 may be a single InGaAsP layer or an MQW structure.

  FIG. 14 shows an optical modulator 50 according to a third embodiment of the present invention. The optical modulator 50 includes a silicon optical waveguide layer and a silicon oxide film provided on a silicon substrate 49. The refractive index is changed by changing the carrier concentration of the optical waveguide layer according to a modulation signal applied from the outside. Let it work.

  The optical modulator 50 is manufactured using an SOI (Silicon on Insulator) structure in which a silicon oxide film 42 and an N-type silicon layer 43 are provided on a silicon substrate 49. The N-type silicon layer 43 on the silicon oxide film 42 is processed into a stripe shape in the light guiding direction, and a gate dielectric film 44 is further formed. In this embodiment, the gate dielectric film 44 is a silicon thermal oxide film. Next, a P-type silicon layer is formed on the gate dielectric film 44 and the silicon oxide film 42. The P-type silicon layer is polycrystalline silicon formed by a thermal CVD method. The P-type silicon layer is also processed into a stripe shape in the light guiding direction, and the exposed portion of the gate dielectric layer 44 is removed by etching, so that the N-type silicon 43 / gate dielectric 44 / P shown in FIG. An optical waveguide having a laminated structure of the mold silicon layer 45 is formed. Further, a silicon oxide film 46 is formed by a thermal CVD method. The silicon oxide films 42 and 46 disposed with the optical waveguide interposed therebetween function as a first cladding layer and a second cladding layer, respectively. Further, a contact hole is formed in the silicon oxide film 46, and the P electrode 48 and the N electrode 47 are connected to the P type silicon layer 45 and the N type silicon layer 43, respectively. Further, a multilayer coating film for adjusting the reflectance is formed on the end face 49 (open surface) of the optical modulator 50 (see FIG. 12).

  Furthermore, the Bragg reflector can also be configured in the optical modulator according to the present embodiment. That is, a Bragg reflector that reflects the laser light may be configured by the difference in refractive index between the silicon oxide film 42 functioning as the first cladding layer and the silicon. Specifically, a diffraction grating in which thin silicon wires are periodically arranged is formed on the silicon oxide film 42 and further embedded with a silicon oxide film (see FIGS. 13A and 13B). Furthermore, an optical modulator having a Bragg reflector can be manufactured by forming an optical waveguide having a laminated structure of the N-type silicon 43 / gate dielectric 44 / P-type silicon layer 45 by a thermal CVD method. .

  FIG. 15 is a schematic diagram showing a BB cross section of the optical modulator 50. P electrode 48 and N electrode 47 are connected to P type silicon layer 45 and N type silicon layer 43 through contact hole 51. When a positive voltage is applied to the P electrode 48 by a modulation signal applied from the outside while the N electrode 47 is grounded to the ground potential, holes accumulate at the interface on the P silicon layer 45 side of the gate dielectric 44. Electrons are accumulated at the interface on the N silicon side. On the other hand, when a negative voltage is applied to the P electrode 48, a depletion region where carriers are discharged is formed at both interfaces of the gate dielectric film 44. That is, the voltage applied to the P electrode 48 changes the carrier concentration in the vicinity of the gate dielectric film 44 serving as an optical waveguide, resulting in a change in refractive index. As a result, the light transmittance of the optical modulator 50 changes, and the input light can be modulated.

  In the optical modulator 50, the refractive index increases when holes and electrons are accumulated near the gate dielectric film 44, and the refractive index decreases when a depletion region is formed. Therefore, the operating temperature is controlled so that the light transmittance of the optical modulator 50 is minimized when the N electrode 47 is at the ground potential and the negative voltage is applied to the P electrode 48, and the positive electrode is applied to the P electrode 48. In this case, if the light transmittance is set to the maximum, normal phase mode operation can be obtained.

  FIG. 16 schematically shows the configuration of a wavelength tunable laser module 60 incorporating the optical modulator 1 according to the present invention. Inside the metal package 61, a laser chip 62, an optical isolator 63, and the optical modulator 1 are arranged. Each device is disposed on a conductive stage 66 bonded to the package 61 via a Peltier cooler 67. The stage 66 is installed at the ground potential. The laser chip 71 is die-bonded on the heat sink 71 in order to improve heat dissipation. The N electrode of the laser chip is electrically connected to the stage 66 via the heat sink 71 and is at ground potential. On the other hand, the P electrode side is connected to an external current source 69, and the laser chip 62 oscillates in the CW mode. The laser chip 62 is a wavelength tunable laser, and oscillates at a predetermined wavelength under external control (not shown).

  Laser light emitted from the laser chip 62 is input to the optical modulator 1 via the optical isolator 63. The optical isolator 63 is disposed so as not to return the reflected light from the incident end face of the optical modulator 1 and the laser light that is multiply reflected inside the optical modulator 1 to the laser chip 62. This is because when the light emitted from the laser chip 62 returns to the inside of the laser chip 62, the laser oscillation becomes unstable, resulting in fluctuations in wavelength and fluctuations in output. In the optical modulator 1 according to the present invention, the multiple reflection within the reflection mirror at the end face and inside the modulator is an indispensable element, and the return light from the input side of the optical modulator to the laser chip 62 is inevitable. Therefore, the optical isolator 63 between the laser chip 62 and the optical modulator 1 is an indispensable component.

  The optical modulator 1 is electrically connected to the stage 66 through the heat sink 72, and the N electrode is installed at the ground potential. Further, the P electrode is connected to an external modulation circuit, and a modulation signal 8 is applied. The laser beam input to the optical modulator 1 is modulated and output by a modulation signal 8 applied from the outside. The signal light output from the optical modulator 1 is input to the optical fiber 65 and transmitted to the receiving side.

  Between each device, a lens 64 for optical coupling is disposed. Further, as described above, the temperature during operation of the optical modulator 1 is important for setting the operation mode, and is precisely controlled by the Peltier cooler 67. Furthermore, the FSR, which is the interval between the resonance frequencies of the optical modulator 1, can also be adjusted by changing the temperature of the modulator.

FIG. 17 is a diagram schematically showing the relationship between the oscillation frequency of the wavelength tunable laser and the transmittance of the optical modulator 1 when the FSR of the optical modulator 1 is matched with the ITU grid standard. As for the optical wavelength used in the wavelength division multiplexing (WDM) communication system, an ITU grid interval which is an international standard is defined, and it is extremely important to match the FSR of the optical modulator 1 with this standard. In order to make the FSR coincide with the ITU grid interval, the resonator length L that determines the interval of the resonance frequency of the optical modulator 1 is set to be the frequency interval determined by the standard. For example, since L = c ₀ / 2ω ₀ (ω ₀ is FSR), L = 1500 μm when the grid interval is 100 GHz.

  As described above, setting the operating point at which the transmittance of the optical modulator 1 is minimized at the optical frequency of the laser light includes the method of applying an offset voltage, the method of adjusting the operating temperature of the optical modulator, or both. It is possible by carrying out in combination. When the FSR matches the ITU grid standard, if the operating point of the optical modulator 1 is set at one wavelength, the operating point can be set at all wavelengths defined by the ITU standard. Will be. That is, even if the wavelength of the wavelength tunable laser is changed to another wavelength that conforms to the ITU standard, it means that there is no need to change the setting of the operating point of the optical modulator 1. FIG. 17 schematically shows this. Thereby, the usability of the wavelength tunable laser module 60 including the optical modulator 1 can be improved in each stage.

The preferred embodiments of the present invention have been described in detail above, but the optical modulator and the wavelength tunable laser module according to the present invention are not limited to the above-described embodiments, and the present invention described in the claims. Various modifications and changes can be made within the scope of the present invention.

It is a schematic diagram which shows the optical modulator which concerns on this invention. It is the schematic diagram which showed the light transmission characteristic of the optical modulator which concerns on this invention with respect to the optical frequency. It is the schematic diagram which showed the light transmission characteristic of the optical modulator which concerns on this invention with respect to the optical frequency. It is the graph which showed intensity ratio (relative light intensity) of output light to input light of a light modulator concerning the present invention to reverse bias. The output waveform of the optical modulator according to the present invention and the corresponding frequency fluctuation Δω are shown. The output waveform of the optical modulator according to the present invention and the corresponding frequency fluctuation Δω are shown. It is explanatory drawing which shows the change of the waveform after transmission of the optical pulse output from the optical modulator which concerns on this invention. 3 shows an eye pattern of 10 Gbps signal light modulated by the optical modulator according to the present invention. The eye pattern after transmission of the signal light modulated by the optical modulator according to the present invention is shown. The eye pattern after transmission of the signal light modulated by the optical modulator according to the present invention is shown. 1 is a schematic diagram illustrating an optical modulator according to a first embodiment of the present invention. It is a schematic diagram which shows the AA cross section of the optical modulator which concerns on 1st Example of this invention. It is a schematic diagram which shows the optical modulator which concerns on the 2nd Example of this invention. 7 shows an optical modulator according to a third embodiment of the present invention. It is a schematic diagram which shows the BB cross section of the optical modulator 50 which concerns on the 3rd Example of this invention. It is a schematic diagram which shows the structure of the wavelength tunable laser module incorporating the optical modulator which concerns on this invention. It is the schematic diagram which showed the relationship between the oscillation frequency of a wavelength-tunable laser and the transmittance | permeability of an optical modulator at the time of making FSR of the optical modulator based on this invention correspond to the specification of ITU grid.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical modulator 2 N type clad layer 3 P type clad layer 4 Optical waveguide layer 5 Reflective mirror 6 P electrode 7 N electrode 22 N type InP layer 23 Optical waveguide layer 24 P type InP layer 25 P electrode 26 N electrode 30 Coating film 31 Diffraction grating 33 Antireflection film 42 Silicon oxide film 43 N-type silicon layer 44 Gate dielectric film 45 P-type silicon layer 46 Silicon oxide film 47 P electrode 48 N electrode 60 Wavelength tunable laser module 62 Laser chip 63 Optical isolator

Claims (12)

An optical modulator for superimposing a signal on a laser beam,
An optical waveguide layer for guiding the laser light;
A first clad layer and a second clad layer which are provided with the optical waveguide layer interposed therebetween and have a refractive index smaller than that of the optical waveguide layer;
Two reflectors provided opposite to each other for multiple reflection of the laser light propagating through the optical waveguide layer;
An electrode disposed between the two reflectors to which a modulation signal for changing a refractive index of the optical waveguide layer is applied;
The electrodes, the the modulation signal following the refractive index of said optical waveguide layer is relatively high, the counter to two so that the strength is high in the signal light output to the outside of the reflector provided optical modulator, wherein that you are biased. The two reflectors are:
The structure according to claim 1, comprising a cleaved surface formed at an end of the optical waveguide layer, or a mirror surface on which the cleaved surface is coated with a coating for adjusting reflectivity. Light modulator. The two reflectors are:
The optical modulator according to claim 1, wherein the optical modulator is a Bragg reflector having a diffraction grating that interferes with laser light propagating through the optical waveguide layer. The optical modulator according to claim 3, further comprising an antireflection film for preventing reflection of the laser light on a cleaved surface provided on both end faces of the optical waveguide layer. The first cladding layer is made of a compound semiconductor crystal selected from an InGaAsP mixed crystal system containing InP that is transparent to laser light propagating through the optical waveguide layer,
The second cladding layer is made of a compound semiconductor crystal selected from the InGaAsP mixed crystal system containing InP.
The optical waveguide layer is
5. The optical modulator according to claim 1, further comprising a compound semiconductor layer selected from the InGaAsP mixed crystal system. The optical waveguide layer is
A multiple quantum well (MQW) structure in which a barrier layer made of a compound semiconductor crystal selected from the InGaAsP mixed crystal system containing InP and a well layer made of the compound semiconductor crystal selected from the InGaAsP mixed crystal system are stacked. The optical modulator according to claim 5, further comprising: The optical waveguide layer is
A barrier layer made of a compound semiconductor crystal selected from an AlGaInAs mixed crystal system that is transparent to the laser light propagating through the optical waveguide layer, a well layer made of a compound semiconductor crystal selected from the InGaAsP mixed crystal system, The optical modulator according to claim 5, wherein the optical modulator has a multiple quantum well (MQW) structure in which a plurality of layers are stacked. The first cladding layer has a first conductivity;
The second cladding layer has a second conductivity;
2. The refractive index of the optical waveguide layer is changed by applying a modulation signal having a reverse bias to two electrodes connected to the first cladding layer and the second cladding layer, respectively. 8. The optical modulator according to any one of 7 to 7. The first cladding layer and the second cladding layer are made of a dielectric layer that is transparent to laser light propagating through the optical waveguide layer,
The optical waveguide layer is provided between the first cladding layer and the second cladding layer, and has a configuration in which a P-type silicon layer, a dielectric layer, and an N-type silicon layer are stacked.
5. The optical modulator according to claim 1, wherein two electrodes to which the modulation signal is applied are connected to the P-type silicon layer and the N-type silicon layer, respectively. The two reflectors are:
10. The optical modulator according to claim 1, wherein a free spectral range (FSR), which is an interval between adjacent resonance frequencies of the opposing reflectors, coincides with a predetermined interval. . A tunable laser that can change the wavelength of the output light,
An optical modulator according to any one of claims 1 to 10,
A tunable laser module comprising: The wavelength tunable laser module according to claim 11, further comprising an optical isolator for attenuating return light to the wavelength tunable laser between the wavelength tunable laser and the optical modulator.

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