JP2010206768A - Optical transmission apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission apparatus capable of making a desired extinction ratio compatible with long-distance fiber transmission. <P>SOLUTION: A semiconductor laser 13 outputs a modulated optical signal obtained by directly modulating frequency of a carrier wave according to a baseband electric signal 11. An optical filter 14 is composed of an asymmetric Mach-Zehnder interferometer and converts a frequency modulation component of the modulated optical signal into an amplitude modulation signal. An FSR of the asymmetric Mach-Zehnder interferometer meets an illustrated formula wherein an extinction ratio of a transmission optical signal 15 is denoted by ER, output of the transmission optical signal 15 normalized with a strength of the modulated optical signal 17 is denoted by P, a spectrum half-value width of the modulated optical signal 17 is denoted by W, a suppression ratio of a residual sideband included in the transmission optical signal 15 is denoted by S, optical frequency detuning of a space optical signal of the modulated optical signal 17 is denoted by f<SB>0</SB>, and optical frequency detuning of a spectrum center frequency of the modulated optical signal 17 is denoted by f<SB>c</SB>. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical transmission apparatus and method, and more particularly to an optical transmission apparatus and method for directly modulating a semiconductor laser according to a baseband electrical signal.

  In recent years, with the widespread use of broadband connections, traffic has increased rapidly, and access methods and services have been diversified. In order to cope with this, the development of an optical fiber communication network is proceeding, and an upgrade to a large capacity communication network is in progress. In the maintenance of an optical fiber communication network, the cost of installing an optical fiber accounts for a considerable portion. In order to suppress the installation cost, it is preferable to use the existing optical fiber network as it is. Even when the transmission speed is increased to increase the capacity, the use of an expensive dispersion compensating fiber should be avoided.

  For example, many optical fiber communication networks between cities that are currently commercialized have a transmission speed of 2.5 Gb / s and a standard single mode fiber (SMF) transmission span of 200 km or less. In order to upgrade the transmission speed to 10 Gb / s, it is necessary to newly realize a 10 Gb / s optical transmission apparatus that has dispersion tolerance and can transmit about SMF 200 km without a dispersion compensating fiber.

  So far, several systems have been developed as 10 Gb / s optical transmission technologies. One of them is a duobinary transmission system using a lithium niobate (LN) optical modulator. This transmission method is excellent in dispersion tolerance. However, the size of the LN optical modulator is as large as several centimeters and the driving power is large. For this reason, since it cannot be incorporated in a small module such as XFP (10 Gbps (X) Form-factor Pluggable) and is expensive, its practical application is limited.

  As a small-sized optical modulator with low driving power, a semiconductor optical modulator using a semiconductor electro-optical absorption effect has been developed. However, due to the chirp characteristics in which the frequency of the carrier wave fluctuates with time, the maximum transmission distance that can be transmitted without dispersion compensation in a semiconductor optical modulator using the semiconductor electro-optical absorption effect is about 40 to 80 km of SMF. is there. In addition, a semiconductor optical modulator using a semiconductor electro-optical absorption effect has a problem in that the wavelength range that can be used is narrow, and therefore, when it is used for high-density wavelength division multiplexing (DWDM), the element selection / adjustment cost becomes expensive. is there.

  On the other hand, if the semiconductor laser is directly modulated, it is smaller than the above-mentioned semiconductor optical modulator and the driving power can be lowered. Further, since an external modulator is not necessary, the price is low. However, the direct modulation of the semiconductor laser has a problem that the transient frequency chirp is large, and the transmission of about 10 km of SMF is limited due to the fiber dispersion. This transient frequency chirp occurs due to a transient change in carrier density and photon density inside the semiconductor laser when the semiconductor laser is directly modulated with a large amplitude.

  In order to solve the above problem, the semiconductor laser may be subjected to small signal modulation by applying a direct current to high bias several times the threshold value. However, when a semiconductor laser is modulated by a small signal, the extinction ratio of the output optical signal becomes small, which causes another problem that it cannot be used for long-distance optical fiber transmission.

  Thus, methods have been proposed for improving the extinction ratio by utilizing the fact that the modulated optical signal output from the semiconductor laser is frequency-modulated (Patent Documents 1 to 3, Non-Patent Document 1). FIG. 21 shows a basic configuration of an optical transmission device using an optical filter. The semiconductor laser 202 is directly modulated with a modulation current according to the data signal 201. This optical transmission device improves the extinction ratio of the modulated optical signal by using the slope of the transmission characteristic of the optical filter 203. In this method, since the spectrum width of the optical signal is also halved at the same time, dispersion tolerance equivalent to that of the duobinary modulation method can be obtained, and transmission of SMF of 200 km or more has been demonstrated without dispersion compensation (Non-patent Document 1). .

  As described in Non-Patent Document 1, in the above method, the frequency modulation amount at 10 Gb / s, that is, the frequency shift of the mark (“1”) code with respect to the space (“0”) code is related to the fiber dispersion. An optimum value is 4.5 to 5 GHz. Therefore, in order to obtain an extinction ratio of 10 dB or more required as an optical signal, the slope of the optical filter 203 must be about 2 dB / GHz. In Non-Patent Document 1, an etalon resonant optical filter composed of a dielectric multilayer film is used to optimize the transmission characteristics of the optical filter and obtain a large slope.

Japanese Patent Publication No. 8-10848 Japanese Patent No. 3768940 US Pat. No. 6,963,685

I Triple E Photonics Technology Letters, Vol. 18, 385-387 (IEEE Photonics Technology Letters, Vol. 18, No.2, JANUARY 15, 2006, pp.385-387) G. P. Agrawar, Nonlinear Fiber Optics (Yoshioka Shoten, Physics Series), pages 71-77

  However, the etalon resonant optical filter composed of a dielectric multilayer film is large in size and requires an optical axis alignment with the semiconductor laser, and further, an optical isolator for preventing reflection between the semiconductor laser and the optical filter. Need to be inserted. Therefore, in order to further reduce the size and the price of the optical transmission device, an optical filter that is smaller in size and capable of hybrid integration with a semiconductor laser without an isolator is required.

  A single ring resonant optical filter configured on a planar optical waveguide circuit (PLC) is small in size and similar in structure to a semiconductor laser and a waveguide structure. It becomes unnecessary. Furthermore, if the drop port is used, no isolator is required. However, in PLC, since the filter transmission characteristic cannot be optimized by devising the manufacturing process like a dielectric multilayer film, the slope of the transmission characteristic of the drop port in the ring resonant optical filter for 10 Gb / s is It must be a small value of about 1 dB / GHz. That is, in order to obtain an extinction ratio of 10 dB or more using a drop port of a single ring resonance type optical filter, the frequency modulation amount must be about 10 GHz. In this case, the frequency modulation amount is about twice the optimum value, and the frequency shift of the mark code with respect to the space code becomes large. The frequency chirp generated is affected by the fiber dispersion, and the transmission distance is significantly reduced. .

  Further, when the frequency shift of the mark code with respect to the space code is 5 GHz, it is necessary to superimpose the amplitude modulation effect by 5 dB in order to obtain the extinction ratio of 10 dB. For this purpose, it is necessary to lower the direct current bias of the semiconductor laser and perform direct modulation at an operating point close to large signal modulation. This increases the transient frequency chirp as described above, and as a result, long-distance fiber transmission is not possible. That is, in the method using the drop port of a single ring resonance type optical filter, it is difficult to achieve both improvement of the extinction ratio and extension of the transmission distance.

  In view of the above, an object of the present invention is to provide an optical transmission method and an optical transmission apparatus capable of achieving both a desired extinction ratio and long-distance fiber transmission.

を満たすことを特徴とする。 To achieve the above object, an optical transmitter according to the present invention includes a semiconductor laser that directly modulates a baseband electric signal and outputs a modulated optical signal in which the frequency of a carrier wave is modulated, and a frequency modulation component of the modulated optical signal. And an optical filter composed of an asymmetric Mach-Zehnder interferometer that outputs a transmission optical signal, and a free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer is used to quench ER of the transmission optical signal. Ratio, P is the output of the transmitted optical signal normalized by the intensity of the modulated optical signal input to the asymmetric Mach-Zehnder interferometer, W is the spectral half width of the modulated optical signal, and S is the residual included in the transmitted optical signal. suppression ratio of sideband optical frequency detuning of the space optical signal of f ₀ the modulated optical signal, in the spectrum of the f _c the modulated optical signal As an optical frequency detuning of the frequency,

It is characterized by satisfying.

  The optical communication system of the present invention includes the optical transmission device of the present invention and an optical reception device optically connected to the optical transmission device via an optical transmission path.

を満たすことを特徴とする。 The optical transmission method of the present invention directly modulates a baseband electric signal with a semiconductor laser to generate a modulated optical signal in which the frequency of a carrier wave is modulated, and the frequency modulation component of the modulated optical signal is obtained from an asymmetric Mach-Zehnder interferometer. A transmission optical signal is generated by converting it into an amplitude modulation signal by an optical filter, and the free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer is ER is the extinction ratio of the transmission optical signal, and P is the asymmetric Mach-Zehnder The output of the transmitted optical signal normalized by the intensity of the modulated optical signal input to the interferometer, W is the spectrum half-value width of the modulated optical signal, and S is the suppression ratio of the residual sideband included in the transmitted optical signal optical frequency detuning of the f ₀ space optical signal of the modulated optical signal, the optical frequency Dichunin spectrum center frequency of f _c the modulated optical signal As,

It is characterized by satisfying.

  The optical transmission apparatus and method of the present invention can achieve both a desired extinction ratio and long-distance optical fiber transmission.

1 is a block diagram showing a basic configuration of an optical communication system using an optical transmission apparatus according to an embodiment of the present invention. The graph which shows the relationship between an optical frequency and light intensity. The typical top view which shows the schematic structure of the optical filter which consists of an asymmetric Mach-Zehnder interferometer used in a 1st Example. (A) is a graph showing an amplitude waveform of a modulated optical signal, and (B) is a graph showing a frequency modulation waveform. (A) is a back-to-back (B2B) eye pattern of the transmitted optical signal, (B) is an eye pattern after 80 km transmission, (C) is an eye pattern after 100 km transmission, and (D) is an eye pattern after 150 km transmission. pattern. (A) is a graph showing the relationship between the spectrum of a modulated optical signal and the transmission characteristics of an optical filter comprising an asymmetric Mach-Zehnder interferometer, and (B) is the transmission of an optical filter comprising a transmitted optical signal and an asymmetric Mach-Zehnder interferometer. The graph which shows the relationship with a characteristic. (A) is a back-to-back (B2B) eye pattern of the transmitted optical signal, (B) is an eye pattern after 80 km transmission, (C) is an eye pattern after 100 km transmission, and (D) is an eye pattern after 150 km transmission. pattern. (A) is a graph showing the relationship between the spectrum of a modulated optical signal and the transmission characteristics of an optical filter comprising an asymmetric Mach-Zehnder interferometer, and (B) is the transmission of an optical filter comprising a transmitted optical signal and an asymmetric Mach-Zehnder interferometer. The graph which shows the relationship with a characteristic. (A) is a back-to-back (B2B) eye pattern of the transmitted optical signal, (B) is an eye pattern after 80 km transmission, (C) is an eye pattern after 100 km transmission, and (D) is an eye pattern after 150 km transmission. pattern. (A) is a graph showing the relationship between the spectrum of a modulated optical signal and the transmission characteristics of an optical filter comprising an asymmetric Mach-Zehnder interferometer, and (B) is the transmission of an optical filter comprising a transmitted optical signal and an asymmetric Mach-Zehnder interferometer. The graph which shows the relationship with a characteristic. Graph showing the relationship between the light output and the extinction ratio for the optical frequency detuning f _0. The graph which shows the relationship between the branching ratio of a directional coupler, the dynamic range of an asymmetric Mach-Zehnder interferometer, and the optical output of a transmission optical signal. The typical top view which shows the schematic structure of the optical filter which consists of an asymmetric Mach-Zehnder interferometer, a wavelength plate, and a reflective mirror used in 2nd Example. FIG. 14 is a schematic plan view showing an optical filter having a filter for TM polarized light in addition to the configuration shown in FIG. 13. The figure which shows the cross-section in the position of AB of FIG. The graph which shows the structure birefringence normalized with respect to waveguide width. The graph which shows the relationship between an optical frequency and the transmittance | permeability. The figure which shows the spectrum of a modulation | alteration optical signal and a transmission optical signal. Back-to-back (B2B) eye pattern of the transmitted optical signal. (A) is an eye pattern after 100 km transmission, (B) is an eye pattern after 150 km transmission, and (C) is an eye pattern after 200 km transmission. 1 is a block diagram showing a basic configuration of an optical transmission device using an optical filter.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a functional configuration of an optical communication system according to an embodiment of the present invention. The optical transmitter 10 includes an optical filter 14 including a drive circuit 12, a semiconductor laser 13, and an asymmetric Mach-Zehnder interferometer. The optical transmitter 10 and the optical receiver 20 are connected via an optical transmission path 30 such as an optical fiber.

  The drive circuit 12 receives the baseband electrical signal 11 and generates a modulation current 16. The semiconductor laser 13 is directly modulated with a modulation current superimposed on a DC bias current larger than the oscillation threshold. At this time, the modulated optical signal 17 output from the semiconductor laser 13 is frequency-modulated by the nonlinear gain of the active layer of the semiconductor laser 13 and the effect of plasma oscillation.

  The frequency-modulated modulated optical signal 17 output from the semiconductor laser 13 is input to an optical filter 14 composed of an asymmetric Mach-Zehnder interferometer. The optical filter 14 converts the frequency modulation component of the modulated optical signal 17 into an amplitude modulated optical signal. The optical filter 14 sends a transmission optical signal (amplitude modulated optical signal) 15 converted to amplitude modulation to the optical transmission line 30.

  The transmission optical signal 15 output from the optical transmission device 10 propagates through the optical transmission path 30 and is then input to the optical reception device 20. The optical receiver 20 includes a light receiving element that converts a transmission optical signal received from the optical transmission path 30 into an electric signal, and an electronic circuit that processes a reception signal output from the light receiving element.

  FIG. 2 shows the relationship between the optical frequency and the light intensity. The horizontal axis represents the optical frequency, and the vertical axis represents the logarithm of the light intensity. In FIG. 2, the broken line represents the optical frequency spectrum of the modulated optical signal 17 output from the semiconductor laser 13, and the alternate long and short dash line represents the transmission characteristic of the optical filter 14 configured with an asymmetric Mach-Zehnder interferometer. The solid line represents the frequency spectrum of the transmission optical signal 15. The spectrum of the transmission optical signal 15 is formed by suppressing the low frequency component in the spectrum of the modulated optical signal 17 by the transmission characteristics of the optical filter 14.

  The spectrum of the transmission optical signal 15 includes a main signal 151 and a residual sideband 152. The main signal 151 contributes to the fiber transmission of the baseband electrical signal 11. On the other hand, the residual sideband 152 causes degradation of transmission signal quality due to the dispersion effect of the optical fiber. In order to suppress the deterioration of the transmission signal quality, it is necessary to suppress the residual sideband 152 sufficiently and sufficiently. In FIG. 2, the suppression ratio S is given by the transmittance of the optical filter 14 at the optical frequency at which the light intensity of the residual sideband 152 is maximum.

  In the optical frequency spectrum of the modulated optical signal 17, the effects of frequency modulation and frequency chirp generated due to the dynamics of the semiconductor laser 13 are superimposed on the Fourier transform of the signal component that changes most rapidly in the baseband electrical signal 11. Spectral shape. Here, the adjacent 3-bit signal pattern that changes in the shortest time in the baseband electric signal 11 is 010 or 101. Each of these signal patterns appears with a probability of 1/8.

  When the semiconductor laser 13 is in a small signal modulation operation and the modulation band due to the effects of nonlinear gain and parasitic capacitance is close to the bit rate of the baseband electric signal 11, the waveform of the relaxation oscillation frequency of the semiconductor laser 13 is the bit rate. It can be approximated by a Gaussian function having the half value width of the reciprocal. For example, when the bit rate is 10 Gb / s, the pulse waveform corresponding to the signal pattern of 010 can be approximated by a Gaussian waveform with a half width of 100 picoseconds, and in the case of the Fourier transform limit (Non-Patent Document 2), the frequency spectrum The shape is also a Gaussian function, and its half-value width is 4.4 GHz.

Since the modulated optical signal 17 is frequency-modulated, the spectrum half width W of the modulated optical signal 17 becomes larger than the value of the Fourier transform limit. The space (“0”) and mark (“1”) shown in FIG. 2 are the space (“0”) and mark (“1”) of the modulated optical signal 17 from the semiconductor laser 13 that has undergone frequency modulation. The average value of the corresponding optical frequencies. Further, f ₀ shown in FIG. 2 is a difference (detuning) between the frequency of the space (“0”) optical signal of the modulated optical signal 17 and the frequency at which the transmittance of the optical filter 14 is minimized, and f _c is The difference (detuning) between the spectral center frequency of the modulated optical signal 17 and the frequency at which the transmittance of the optical filter 14 is minimized. The extinction ratio ER is given by the transmittance of the optical filter 14 at the average value of the optical frequencies corresponding to the space (“0”) and mark (“1”) of the modulated optical signal 17.

ただし、Ｗは、変調光信号１７のスペクトル半値幅である。 The frequency spectrum I (f) of the modulated optical signal 17 output from the semiconductor laser 13 is expressed by the following formula 1 when the shape can be approximated by a Gaussian function.

Here, W is the spectral half width of the modulated optical signal 17.

式２におけるＦＳＲは、非対称マッハツェンダ干渉計のフリー・スペクトラル・レンジであり、透過率が最小となる周波数を原点としている。 Further, the transmission characteristic H (f) of the optical filter 14 composed of an asymmetric Mach-Zehnder interferometer can ignore the loss of the interferometer, and the branching ratio of the duplexer / multiplexer constituting the interferometer is 0. In the case of 5, it is represented by the following formula 2.

The FSR in Equation 2 is the free spectral range of the asymmetric Mach-Zehnder interferometer, and the origin is the frequency at which the transmittance is minimum.

In Equation 2, when the cosine function is approximated by a power expansion near f = 0, the spectrum P _s (f) of the residual sideband output from the optical filter 14 is expressed by Equation 3 below.

The frequency at which P _s (f) is maximized is calculated, and this is substituted for the transmission characteristic H (f) of the asymmetric Mach-Zehnder interferometer approximated by a power expansion, so that the residual sideband suppressed by the asymmetric Mach-Zehnder interferometer When the condition that the amount of suppression is greater than the suppression ratio S required for fiber transmission of a high-quality optical signal is obtained, the conditional expression is expressed by the following expression 4.

ここで、光出力Ｐは、送信光信号１５の主信号１５１の光強度を、光フィルタ１４に入力した変調光信号１７の光強度で規格化した値である。 On the other hand, from FIG. 2, the extinction ratio ER is calculated by calculating Equation 2 for the frequency of the modulated optical signal 17 in the space (“0”) optical signal and the frequency of the modulated optical signal 17 in the mark (“1”). When a condition for satisfying the optical output P is obtained, the conditional expression is expressed by the following expression 5.

Here, the optical output P is a value obtained by standardizing the optical intensity of the main signal 151 of the transmission optical signal 15 with the optical intensity of the modulated optical signal 17 input to the optical filter 14.

From Equations 4 and 5, the FSR conditions that the optical filter 14 should satisfy in order to achieve high-quality optical fiber transmission while sufficiently suppressing the residual sideband while obtaining a high optical output and extinction ratio are as follows: It is represented by

  In the present embodiment, the semiconductor laser 13 outputs a modulated optical signal 17 that is directly modulated according to the baseband electrical signal 11 and whose carrier frequency is modulated. The optical filter 14 is composed of an asymmetric Mach-Zehnder interferometer, converts the frequency modulation component of the modulated optical signal 17 into an amplitude modulated signal, and outputs a transmission optical signal 15. The free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer satisfies the relational expression (6). By using the optical filter 14 having such an FSR, a desired light output and a desired extinction ratio can be obtained, and the residual sideband can be sufficiently suppressed. Therefore, in this embodiment, it is possible to achieve both improvement in the extinction ratio and high-quality long-distance optical fiber transmission.

  Hereinafter, description will be made using examples. First, the first embodiment will be described. The modulated optical signal 17 is encoded using a binary baseband electrical signal 11 having a bit rate of 10 Gb / s, and is input to an optical filter 14 including a Mach-Zehnder interferometer. The baseband binary signal is a binary NRZ (Non Return Zero) signal. In the first embodiment, a signal having a transmission capacity of 10 Gb / s can be transmitted through a fiber in a wavelength band of 1.5 μm. A standard single mode fiber having a dispersion value of 17 psec / nm / km is used for the optical transmission line 30. The length of the optical transmission line 30 is 80 km to 150 km, and the optical transmission line 30 does not compensate for dispersion. The bit rate of the baseband electrical signal 11 is not limited to 10 Gb / s, and may be 2.5 Gb / s or 40 Gb / s.

  A binary baseband electrical signal 11 having a bit rate of 10 Gb / s is input to the drive circuit 12. The semiconductor laser 13 is directly modulated by the modulation current 16 output from the drive circuit 12. The optical filter 14 composed of an asymmetric Mach-Zehnder interferometer converts the frequency modulation component of the modulated optical signal 17 output from the semiconductor laser 13 into amplitude modulated light, and outputs a transmission optical signal 15. The transmission optical signal 15 is a signal encoded using a binary baseband electric signal 11 having a bit rate of 10 Gb / s.

The semiconductor laser 13 is applied with a direct current larger than the oscillation threshold value from a direct current bias circuit, and is directly modulated by a modulation current 16 applied in a superimposed manner on the direct current. As the semiconductor laser 13, a commercially available distributed feedback semiconductor laser (DFB-LD) can be used. Specifically, a λ / 4 shift distributed feedback semiconductor laser (DFB-LD) in which both end faces are subjected to antireflection coating can be used for the semiconductor laser 13. Regarding the parameters in the semiconductor laser 13, the resonator length is 300 μm, the grating coupling constant κ is 7000 m ⁻¹ , the active layer width is 2 μm, the active layer thickness is 0.05 μm, the optical confinement factor is 0.1, and the α parameter is 4 The differential gain coefficient is 6.5 × 10 ⁻²⁰ m ² , the nonlinear gain coefficient is 4.5 × 10 ⁻²³ m ³ , and the resonator internal loss is 3000 m ⁻¹ .

  Each of the above parameters is one design example. Each parameter only needs to be designed according to the specifications of the optical communication system and the characteristics of the optical transmitter 10, so that it is realized as a distributed feedback semiconductor laser having an active layer of a normal multilayer quantum well or strained quantum well structure. It is good also as values other than the value illustrated above in the range which can do. In the band of the drive circuit 12, the cutoff frequency is 10 GHz.

FIG. 3 schematically shows a schematic structure of the optical filter 14 composed of an asymmetric Mach-Zehnder interferometer used in the first embodiment of the present invention. The optical filter 14 is a ring resonance type optical filter. The optical filter 14 includes two directional couplers 142 and 145 and two waveguides 143 and 144 having different lengths. The optical filter 14 is configured as a planar optical waveguide circuit (PLC) using a silicon oxide / silicon nitride (SiON) layer formed on a silicon substrate as a core and silicon oxide (SiO ₂ ) as a cladding layer.

  The optical signal input to the optical input unit 141 is branched into two by the first directional coupler 142 and guided to the short waveguide 143 and the long waveguide 144. The optical signals guided using the short waveguide 143 and the long waveguide 144 are combined by the second directional coupler 145 and output from the optical output unit 146.

  Here, the free spectral range (FSR) of the optical filter 14 composed of an asymmetric Mach-Zehnder interferometer is given by the reciprocal of the propagation time difference of the optical signal between the short waveguide 143 and the long waveguide 144. In the first embodiment, the FSR of the optical filter 14 is 15 to 50 GHz, the power branching ratio of the first directional coupler 142 and the second directional coupler 145 is 0.5, and the propagation loss of the waveguide Is ignored.

  The optical filter 14 composed of an asymmetric Mach-Zehnder interferometer is different in principle from a plane incidence type optical filter composed of a dielectric multilayer film, and in principle, reflected light does not return to the light input unit 141. For this reason, an optical isolator between the semiconductor laser 13 and the optical filter 14 becomes unnecessary, and the apparatus can be reduced in size and cost. In addition, the hybrid integration of the semiconductor laser 13 and the ring resonance type optical filter 14 is possible by using an automatic assembly technique of a PLC and a semiconductor laser on a silicon substrate such as flip-chip mounting.

  The operation of the first embodiment will be described below based on specific calculation results. The calculation results obtained by approximating the operation of the distributed feedback semiconductor laser with a single-mode rate equation and analyzing the optical fiber transmission characteristics using the split-step Fourier method are shown below. The specifications of the optical transmission device 10 are such that the extinction ratio of the transmission optical signal 15 is 10 dB and the SMF transmission distance is 80 to 150 km.

  When the semiconductor laser 13 is directly modulated, the modulated optical signal 17 that is an output thereof is frequency-shifted to the high frequency side when the modulation current 16 is increased. From the viewpoint of fiber dispersion, the optimum value of the frequency difference Δf between the optical frequency of the space code and the optical frequency of the mark code is 4 to 5 GHz or less. This is because the spectral width that is the Fourier transform limit of an optical pulse with a time width of 100 picoseconds corresponding to a 010 pattern of an NRZ signal of 10 Gb / s is 4.4 GHz. This is because it is affected by dispersion (Non-Patent Document 2).

  Further, in order to obtain an extinction ratio of 10 dB, the semiconductor laser 13 is DC-biased so that the carrier frequency at the time of space coding is higher by about 1 to 3 GHz than the frequency at which the transmission of the optical filter 14 is minimized. Accordingly, since the carrier frequency at the time of mark coding is located on the high frequency side by the mark / space frequency difference Δf, there is almost no effect of dispersion compensation performed by the optical filter 14 comprising an asymmetric Mach-Zehnder interferometer. .

  In the first embodiment, the current amplitude of the modulation current 16 of the semiconductor laser 13 is set to 16 milliamperes, and the direct current bias of the semiconductor laser 13 is set to 80 milliamperes as a current value at the time of a space code. 4A shows an amplitude waveform of the modulated optical signal 17, and FIG. 4B shows a frequency modulation waveform. Referring to the figure, it can be seen that the waveform has almost no relaxation oscillation and transient frequency chirp due to the non-linear gain and the band limiting effect of the drive circuit 12. Referring to FIG. 4B, it can be seen that the mark / space frequency difference Δf is about 5 GHz.

  FIGS. 5A to 5D show eye patterns of the transmission optical signal 15 when the FSR of the asymmetric Mach-Zehnder interferometer is 50 GHz. FIG. 5A shows a back-to-back (B2B) eye pattern of the transmission optical signal 15, and FIG. 5B shows an eye pattern after 80 km transmission. FIG. 5C shows an eye pattern after 100 km transmission, and FIG. 5D shows an eye pattern after 150 km transmission.

の上限は、マーク／スペースの周波数差Δｆがガウス関数の時間応答として与えられた場合、下限は、フーリエ変換限界である。第一実施例の動作条件は、式７を満たしている。 6A shows the relationship between the spectrum of the modulated optical signal 17 and the transmission characteristic of the optical filter 14, and FIG. 6B shows the relationship between the spectrum of the transmission optical signal 15 and the transmission characteristic of the optical filter 14. Show. Referring to FIG. 6A, it can be seen that the spectrum of the modulated optical signal 17 can be approximated by a Gaussian curve having a half-value width of 5 GHz under the operating conditions of the first embodiment. This half-value width satisfies the following formula 7 between the bit rate B and the mark / space frequency difference Δf when the semiconductor laser 13 is operating under conditions without relaxation oscillation or transient chirp.

When the mark / space frequency difference Δf is given as a time response of a Gaussian function, the lower limit is the Fourier transform limit. The operating condition of the first embodiment satisfies Expression 7.

  Further, referring to FIG. 6B, it can be seen that the residual sideband 152 can be reduced by 25 dB (corresponding to the suppression ratio 320) or more by using the FSR value of about 50 GHz. As a result, as shown in FIGS. 5A to 5D, high-quality fiber transmission with almost no penalty is possible up to 150 km while ensuring an extinction ratio of 10 dB in B2B.

  However, when the FSR is 50 GHz, the component of the main signal 151 (FIG. 2) is also attenuated, as can be seen from FIG. In the first embodiment, the average power of the transmission optical signal 15 is about 1.7 m, which is insufficient in practice in consideration of optical loss when assembled in a transceiver module or the like. In order to increase the output power of the transmission optical signal 15, the FSR may be reduced. However, when the FSR is reduced, the suppression ratio of the residual sideband 152 is also reduced. Therefore, it is desirable to select the FSR so as to satisfy both the output of the high transmission optical signal 15 and the sufficient suppression of the residual sideband 152.

Substituting f ₀ = 2.5 GHz, f _c = 5 GHz, ER = 10, P = 0.5, S = 32, = 5 GHz corresponding to the first embodiment into the above formula 6, the following formula 8 is obtained. .
50GHz ≧ FSR ≧ 25GHz (8)
Here, the optical frequency detuning f _c of the spectrum center frequency of the modulated optical signal, in relation to the spectral half-width of the modulated optical signal 17 is given by the following equation 9.

  Referring to Equation 8, it can be seen that the FSR can be reduced to 25 GHz for higher output. FIGS. 7A to 7D show eye patterns of a transmission optical signal when the FSR is 25 GHz. 8A shows the relationship between the spectrum of the modulated optical signal 17 and the transmission characteristics of the optical filter 14 when the FSR is 25 GHz, and FIG. 8B shows the spectrum and light of the transmission optical signal 15. The relationship with the transmission characteristic of the filter 14 is shown. When the FSR is 25 GHz, as can be seen from FIG. 8, the transmission loss of the optical filter 14 can be significantly reduced while obtaining 59 (18 dB) as the suppression ratio S of the residual sideband. That is, 10 dB can be realized as the extinction ratio of B2B, 5.5 m can be realized as the average optical power of the transmission optical signal 15, and high-quality fiber transmission with almost no penalty can be realized up to 150 km.

  As a comparative example, FIGS. 9A to 9D show eye patterns of transmission optical signals when the FSR is 15 GHz. FIG. 10A shows the relationship between the spectrum of the modulated optical signal 17 and the transmission characteristics of the optical filter 14 when the FSR is 15 GHz, and FIG. 10B shows the spectrum and light of the transmission optical signal 15. The relationship with the transmission characteristic of the filter 14 is shown. FSR = 15 GHz is a condition that does not satisfy the condition of Expression 8.

  When the FSR does not satisfy the relational expression of Expression 8, the residual sideband is not sufficiently suppressed. Referring to FIG. 10, when the FSR is 15 GHz, the residual sideband suppression ratio is only 9.5 (9.8 dB). Since the FSR value is small, the average optical power of the transmission optical signal 15 rises to about 8.9 m. However, since the residual sidebands are not sufficiently suppressed, as shown in FIG. 9, significant waveform deterioration occurs at a transmission distance of 80 km (B), and fiber transmission of high-quality optical signals exceeding 100 km becomes difficult. . From the above, it is concluded that if the suppression ratio S is set to 40 (16 dB) or more, fiber transmission without signal quality deterioration due to the residual sideband is possible.

式１０に、Ｐ＝０．５、ＥＲ＝１０、ＦＳＲ＝２５ＧＨｚを代入すると、下記式１１が得られる。
ｆ_０≧１．３ＧＨｚ （１１）
図１１を参照すると、第一実施例においては、高い消光比と光出力を得るための式１１の条件を満たすことがわかる。 FIG. 11 shows the relationship between the optical frequency detuning f ₀ of the space optical signal of the modulated optical signal 17 and the optical output and extinction ratio when FSR = 25 GHz. Referring to FIG. 11, it can be seen that the light output and the extinction ratio are in a trade-off relationship. By transforming Equation 5, Equation 10 is obtained.

Substituting P = 0.5, ER = 10, and FSR = 25 GHz into Equation 10, the following Equation 11 is obtained.
f ₀ ≧ 1.3 GHz (11)
Referring to FIG. 11, it can be seen that in the first embodiment, the condition of Expression 11 for obtaining a high extinction ratio and light output is satisfied.

  FIG. 12 shows the relationship between the branching ratio of the directional coupler in the optical filter 14, the dynamic range of the asymmetric Mach-Zehnder interferometer, and the optical output of the transmission optical signal 15. When the branching ratio of the first directional coupler 142 or the second directional coupler 145 of the asymmetric Mach-Zehnder interferometer is not 0.5, or when the loss of the long waveguide 144 is large, the direction is equivalent. When the branching ratio of the sex coupler deviates from 0.5, the dynamic range of the asymmetric Mach-Zehnder interferometer and the optical output of the transmission optical signal 15 change as shown in FIG. 12 according to the branching ratio.

  Here, the dynamic range of the asymmetric Mach-Zehnder interferometer is defined by the ratio between the maximum value and the minimum value of the transmitted light intensity, and the horizontal axis in FIG. 12 uses the branching ratio of one directional coupler as a parameter. . The loss of the long waveguide 144 can be replaced by the branching ratio of the directional coupler.

  Referring to FIG. 12, it can be seen that there is almost no decrease in light output when the dynamic range is 20 dB or more. The branching ratio of the directional coupler of the optical filter 14 is preferably selected so that the dynamic range is 20 dB or more. For example, in the asymmetric Mach-Zehnder interferometer, when the loss of the long waveguide 144 cannot be ignored, the branching ratio of any directional coupler may be adjusted to a branching ratio that can realize a dynamic range of 20 dB or more.

  Next, a second embodiment will be described. FIG. 13 schematically shows a schematic structure of an optical filter used in the second embodiment of the present invention. The optical filter 18 includes an asymmetric Mach-Zehnder interferometer 14, a wave plate 185, and a reflecting mirror 186. In the optical filter 18, the modulated optical signal 17 from the semiconductor laser 13 is input to the input port 181, and the transmission optical signal 15 is output from the output port 182.

  The asymmetric Mach-Zehnder interferometer 14 constituting the optical filter 18 includes a first directional coupler 142, a second directional coupler 145, and a short waveguide 143 and a long waveguide 144 constituting two arms. Become. The long waveguide 144 has a structure in which, after the waveguide width is once expanded in a tapered shape, it is narrowed again to the original width. The wave plate 185 is a quarter wave plate. The reflecting mirror 186 is a loop mirror.

Similar to the first embodiment, the optical filter 18 can be configured as a planar optical waveguide circuit (PLC), and can be hybrid-integrated with the semiconductor laser 13. Planar optical waveguide circuit, for example, can be made of combinations of materials such as _SiO 2, SiON, SiN formed on a silicon substrate.

  The principle of basic operation in the second embodiment will be described. The modulated optical signal 17 from the semiconductor laser 13 is TE polarized light, and the optical signal input to the asymmetric Mach-Zehnder interferometer 14 through the input port 181 is also TE polarized light. The asymmetric Mach-Zehnder interferometer 14 operates as an optical filter for TE polarized light, converts the frequency modulation component of the modulated optical signal 17 into an amplitude modulated optical signal, and outputs the converted amplitude modulated light from the first waveguide 183. To do. The optical signal output from the first waveguide 183 is the same as the transmission optical signal 15 output from the optical output unit 146 (FIG. 3) in the first embodiment described above. At this time, an unnecessary spectral component removed by the optical filter for TE polarized light is output from the second waveguide 184. This light is terminated without reflection at the second waveguide 184 so as not to return to the semiconductor laser 13 as a TE polarization component.

  The TE-polarized amplitude-modulated light output from the first waveguide 183 is totally reflected by the loop mirror 186 after passing through the quarter-wave plate 185. The totally reflected light passes through the quarter-wave plate 185 again, and the polarized light is rotated to TM polarized light. The amplitude-modulated light that has become TM-polarized light has a propagation direction opposite to that of TE-polarized amplitude-modulated light, and is input again from the first waveguide 183 to the asymmetric Mach-Zehnder interferometer 14. The asymmetric Mach-Zehnder interferometer 14 operates as an optical filter for TM polarized light, and superimposes the frequency modulation component of the amplitude modulated light into an amplitude modulated optical signal, and outputs the converted amplitude modulated light from the output port 182.

  In FIG. 13, the wave plate 185 is formed of a quarter wave plate and the reflecting mirror 186 is formed of a loop mirror. However, the present invention is not limited to this. Instead of arranging the wave plate 185 as a half wave plate and placing the quarter wave plate 185 on the incident side of the loop mirror 186, a half wave plate is arranged in the loop mirror 186. It is also possible. The reflecting mirror 186 is not limited to the loop mirror, and may be realized by forming a waveguide end face and forming a planar reflecting mirror made of a dielectric, metal, or the like on the waveguide end face.

  In the second embodiment, the optical path length difference for the TE polarized light between the two arms of the asymmetric Mach-Zehnder interferometer 14 and the optical path length difference for the TM polarized light are set to be different by (integer +1/2) times the wavelength. Yes. In this case, the asymmetric Mach-Zehnder interferometer 14 is operating in a cross state with respect to TE polarized light (in FIG. 13, all optical signals input from the input port 181 are output from the first waveguide 183). In this case, the TM polarized light operates in a bar state (in FIG. 13, a state in which all optical signals input from the input port 181 are output from the second waveguide 184). On the other hand, in the bar state with respect to the TE polarized light, the cross state is obtained with respect to the TM polarized light.

  The TM-polarized amplitude-modulated light passing through the asymmetric Mach-Zehnder interferometer 14 is output from the output port 182 instead of the input port 181. Thus, by reciprocating the asymmetric Mach-Zehnder interferometer 14, an effect equivalent to connecting two asymmetric Mach-Zehnder interferometers in series using one asymmetric Mach-Zehnder interferometer can be obtained.

  By the way, unnecessary spectral components removed by the asymmetric Mach-Zehnder interferometer 14 operating as an optical filter for TM polarized light return from the input port 181 to the semiconductor laser 13. However, since the return light is TM polarized light and is orthogonal to the TE polarized light of the oscillation light of the semiconductor laser 13, the influence of the return light on the operation of the semiconductor laser 13 is small. In order to remove the influence of the return light, a mode filter (polarization filter) for removing the TM polarization component may be inserted in front of the input port 181. FIG. 14 shows an optical filter having a mode filter. The mode filter (polarizing filter) 187 is inserted between the asymmetric Mach-Zehnder interferometer 14 and the input port 181. The polarizing filter 187 may be one that absorbs TM polarized light by disposing a metal film on the cladding layer of the waveguide, or comprises a Mach-Zehnder interferometer, a directional coupler, an optical branching and multiplexing circuit, etc. It may be a polarizing beam splitter.

Subsequently, a guideline for setting the optical path length difference for the TE polarized light between the two arms of the asymmetric Mach-Zehnder interferometer 14 and the optical path length difference for the TM polarized light to be different by (integer +1/2) times the wavelength. explain. FIG. 15 shows a cross section of the asymmetric Mach-Zehnder interferometer 14 at the position AB in FIG. A SiO ₂ cladding layer 172 is formed on the silicon substrate 171, and a SiON core layer 173 constituting a long waveguide 144 and a short waveguide 143 is embedded in the SiO ₂ cladding layer 172. The waveguide width W ₂ of the long waveguide 144 is wider than the waveguide width W ₁ of the short waveguide 143.

Here, when the waveguide width is equal to the waveguide thickness, the propagation constant of TE-polarized light and the propagation constant of TM-polarized light are equal. On the other hand, when the waveguide width is wider than the waveguide thickness, the TE-polarized light has an effective refractive index increased because the electric field is polarized in a direction parallel to the silicon substrate, and a structural birefringence is generated because the propagation constant decreases. . The magnitude of this structural birefringence is about 1 to 5 × 10 ⁻⁴ . By utilizing this structural birefringence, the optical path length difference for TE polarization and the optical path length difference for TM polarization between the two arms, that is, the short waveguide 143 and the long waveguide 144, can be expressed as It can be set to be different by an integer +1/2) times.

In the second embodiment, the refractive index difference between the SiON core layer 173 and the SiO ₂ cladding layer 172 is 6%. The waveguide thickness t of all the waveguides is 1.5 μm, and the waveguide width W ₁ of the waveguides excluding the long waveguide 144 near the position AB in FIG. 13 is 2 μm. For the sake of simplicity, the polarization dependence of the directional couplers 142 and 145 will be ignored. FIG. 16 shows the result of calculating the waveguide width dependence of the structural birefringence normalized by the effective refractive index when the waveguide thickness t is 1.5 μm for a refractive index difference of 6%. Referring to FIG. 16, it can be seen that when the waveguide width is 2 μm, the structural birefringence is 0.000231, and when the waveguide width is 3 μm, the structural birefringence is 0.000485.

  As described in the first embodiment, the FSR of the asymmetric Mach-Zehnder interferometer 14 must be a value satisfying Equation 6 in order to obtain a high-quality transmitted optical signal 15. Therefore, in the following description of the second embodiment, the FSR is 25 GHz. The power branching ratio of the first directional coupler 142 and the second directional coupler 145 is 0.5, and the propagation loss of the waveguide is ignored.

Assuming that the effective refractive index of the short waveguide 143 is 1.52, the long waveguide 144 needs to have a longer waveguide length than the short waveguide 143 by 7.89 mm in order to set the FSR to 25 GHz. From this value and the value of the structural birefringence when the waveguide width is 2 μm, TE polarization between the short waveguide 143 and the long waveguide 144 when the waveguide widths of all the long waveguides 144 are all 2 μm. It can be calculated that the optical path length difference with respect to TM and the optical path length difference with respect to TM polarized light differ by 1.82 times the wavelength. On the other hand, for the operation of the second embodiment, the difference between the optical path length difference for TE polarized light and the optical path length difference for TM polarized light needs to be (integer +1/2) times the wavelength, that is, 2.5 times. There is. From the above, the long waveguide 144 of waveguide width W ₂ and a length of that portion, the difference in optical path length difference relative to the TE polarized light and TM polarized light may be designed to be increased 0.68 times the wavelength I understand that. For example, long and a waveguide width W ₂ of the waveguide 144 and 3 [mu] m, guide Namijicho part widening to 3 [mu] m width of the waveguide from 2μm at tapered structure with the value of the form birefringence index of above 2.68mm It becomes.

  The operation of the second embodiment will be described based on a specific transmission simulation. Here, the structure and operating conditions of the semiconductor laser 13 are the same as those in the first embodiment. The bit rate is 10 Gb / s, and the wavelength is in the 1.5 μm band. The optical transmission line 30 is a standard single mode fiber having a dispersion value of 17 psec / nm / km. The length of the optical transmission line 30 is 100 km to 200 km, and the optical transmission line 30 is not compensated for dispersion. .

  FIG. 17 shows the transmission characteristics of various optical filters. FIG. 17 shows the transmission characteristics when the asymmetric Mach-Zehnder interferometer has a one-stage configuration, the transmission characteristics when the two-stage configuration is used, the transmission characteristics of a Gaussian filter, and the transmission characteristics of the ring resonator. Referring to FIG. 17, it can be seen that when the asymmetric Mach-Zehnder interferometer has a one-stage configuration, the transmission characteristics are considerably different from those of the Gaussian optical filter. On the other hand, when two stages of asymmetric Mach-Zehnder interferometers are connected in cascade, it is understood that almost the same transmission characteristics as those of the Gaussian optical filter can be obtained until the relative transmittance normalized by the transmission peak intensity is -20 dB.

  FIG. 18 shows the spectral intensity of the modulated optical signal 17 and the transmitted optical signal 15 output from the output port 182 when the semiconductor laser 13 is subjected to small signal modulation under the above operating conditions in the configuration of FIG. . FIG. 18 also shows the transmission characteristics of the optical filter 18 operating as a two-stage asymmetric Mach-Zehnder. The setting of detuning between the semiconductor laser 13 and the optical filter 18 is as described in the first embodiment.

  The shape of the spectrum of the modulated optical signal 17 is Gaussian as described in the first embodiment. Therefore, only when the transmission characteristic of the optical filter 18 is Gaussian, the spectrum shape of the transmission optical signal 15 is Gaussian. Since the waveform of the optical signal is given by Fourier transform of the spectrum of the optical signal, when the spectral shape of the optical signal is Gaussian, the waveform has a Gaussian function shape. On the other hand, when the spectral shape of the optical signal deviates from the Gaussian shape, the waveform, which is the Fourier transform, has a tail and causes waveform distortion such as intersymbol interference. That is, when the transmission characteristic of the optical filter is a Gaussian type, the highest quality transmission optical signal 15 with minimum waveform distortion and intersymbol interference can be obtained.

  FIG. 19 shows the result of the transmission simulation for the back-to-back (B2B) eye pattern of the transmission optical signal 15. Referring to FIG. 19, it can be seen that an extinction ratio of 10 dB is obtained by back-to-back. Further, it can be seen that the waveform is high quality with almost no waveform distortion and intersymbol interference as compared with the first embodiment.

  FIG. 20 shows the eye pattern of the transmitted optical signal after transmission. FIG. 20A shows an eye pattern after 100 km transmission. FIG. 20B shows an eye pattern after 150 km transmission, and FIG. 20C shows an eye pattern after 200 km transmission. The transmission penalty is 1 dB or less even after 200 km transmission, and it can be seen that good long-distance transmission is possible.

  The difference between the first embodiment and the second embodiment is that the optical filter has a configuration in which two asymmetric Mach-Zehnder interferometers are equivalently connected in cascade. By using a configuration in which two asymmetric Mach-Zehnder interferometers are equivalently connected in cascade, the quality of the transmitted optical signal 15 output from the output port 182 can be improved. In other words, the configuration of the second embodiment can reduce the waveform distortion and intersymbol interference of the transmission optical signal 15 and improve the back-to-back waveform quality compared to the first embodiment. In addition, transmission over a longer distance becomes possible.

  In the second embodiment, an effect equivalent to connecting two asymmetric Mach-Zehnder interferometers in series using one asymmetric Mach-Zehnder interferometer is obtained. As described in the first embodiment, the FSR of the asymmetric Mach-Zehnder interferometer 14 needs to satisfy the condition of the sixth formula and always needs to accurately control the detuning with the wavelength of the semiconductor laser 13. is there. When two asymmetric Mach-Zehnder interferometers are used separately, the monitoring mechanism and the control mechanism for control are all twice required. Further, when two asymmetric Mach-Zehnder interferometers are used separately, the control method becomes very complicated from the viewpoint of convergence. Compact, low-power pluggable optical transceiver modules, especially XFP, have limited mounting space and allowable power consumption, and there are limits to the capacity of processors that can be mounted, so multiple asymmetric Mach-Zehnder interferometers are used. It is virtually impossible. In the second embodiment, since only one asymmetric Mach-Zehnder interferometer is used, no special configuration is required for a monitoring mechanism, a control mechanism, and a control method for controlling detuning or the like. Therefore, it is possible to mount the optical filter 18 capable of improving the waveform quality and long-distance transmission in an optical transceiver module such as XFP.

  Although the present invention has been described based on the preferred embodiment, the optical transmission apparatus and method of the present invention are not limited to the above embodiment, and various modifications and changes can be made to the configuration of the above embodiment. Those subjected to are also included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 10: Optical transmitter 11: Baseband electric signal 12: Drive circuit 13: Semiconductor laser 14: Optical filter which consists of an asymmetric Mach-Zehnder interferometer 15: Transmission optical signal 16: Modulated current 17: Modulated optical signal 18: Asymmetric Mach-Zehnder interferometer Optical filter 20 comprising wave plate and reflecting mirror: optical receiver 30: optical fiber transmission line 141: optical input unit 142: first directional coupler 143: short waveguide 144: long waveguide 145: second Directional coupler 146: optical output unit 151: main signal 152: residual sideband 171: silicon substrate 172: SiO ₂ cladding layer 173: SiON core layer 181: input port 182: output port 183: first waveguide 184 : Second waveguide 185: wave plate 186: reflecting mirror 187: mode filter

Claims (13)

A semiconductor laser that outputs a modulated optical signal that is directly modulated in accordance with a baseband electrical signal and whose carrier frequency is modulated;
An optical filter comprising an asymmetric Mach-Zehnder interferometer that converts a frequency modulation component of the modulated optical signal into an amplitude modulated signal and outputs a transmitted optical signal;
The transmitted optical signal in which the free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer is normalized by ER as the extinction ratio of the transmitted optical signal and P as the intensity of the modulated optical signal input to the asymmetric Mach-Zehnder interferometer. output, W a spectral half width of said modulated optical signal, suppression ratio of vestigial sideband included the S in the transmission optical signal, the optical frequency detuning of the space optical signal of f ₀ the modulated optical signal, f _c As optical frequency detuning of the spectrum center frequency of the modulated optical signal,

An optical transmitter that satisfies the requirements.   The optical transmitter according to claim 1, wherein the asymmetric Mach-Zehnder interferometer includes two directional couplers and two optical waveguides having different lengths.   The optical transmission device according to claim 2, wherein a branching ratio of the directional coupler is given so that a dynamic range of transmission characteristics of the asymmetric Mach-Zehnder interferometer is 20 dB or more.   The optical transmission device according to claim 2 or 3, wherein a free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer is given by a difference in length between the two optical waveguides.   5. The optical transmission device according to claim 2, wherein the asymmetric Mach-Zehnder interferometer is configured by a planar optical waveguide circuit. 6.   The optical transmitter according to claim 5, wherein the asymmetric Mach-Zehnder interferometer is hybrid-integrated with the semiconductor laser. A wave plate connected to one port of the asymmetric Mach-Zehnder interferometer to which the semiconductor laser is not connected and a reflecting mirror;
The optical path length difference for TE polarized light and the optical path length difference for TM polarized light between the two arms of the asymmetric Mach-Zehnder interferometer are different by (integer +1/2) times the wavelength of the semiconductor laser, and the asymmetric Mach-Zehnder interferometer The optical transmission device according to any one of claims 1 to 6, wherein an optical output is extracted from a port to which the semiconductor laser is not connected among ports connected to the semiconductor laser.   The optical transmission device according to claim 7, wherein a polarization filter for removing a TM polarization component is inserted between the asymmetric Mach-Zehnder interferometer and the semiconductor laser.   The optical transmission device according to claim 7 or 8, wherein the reflecting mirror is configured as a loop mirror.   The wave plate is a quarter wave plate, and the quarter wave plate is inserted between one port of the asymmetric Mach-Zehnder interferometer on the side where the semiconductor laser is not connected and the reflecting mirror. The optical transmission device according to any one of claims 7 to 9.   The optical transmission device according to claim 9, wherein the wave plate is a half-wave plate, and the half-wave plate is inserted into the loop mirror.   An optical communication system comprising: the optical transmission device according to claim 1; and an optical reception device optically connected to the optical transmission device through an optical transmission path. A baseband electrical signal is directly modulated by a semiconductor laser to generate a modulated optical signal whose carrier frequency is modulated,
A transmission optical signal is generated by converting the frequency modulation component of the modulated optical signal into an amplitude modulated signal with an optical filter composed of an asymmetric Mach-Zehnder interferometer,
The transmitted optical signal in which the free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer is normalized by ER as the extinction ratio of the transmitted optical signal and P as the intensity of the modulated optical signal input to the asymmetric Mach-Zehnder interferometer. output, W a spectral half width of said modulated optical signal, suppression ratio of vestigial sideband included the S in the transmission optical signal, the optical frequency detuning of the space optical signal of f ₀ the modulated optical signal, f _c As optical frequency detuning of the spectrum center frequency of the modulated optical signal,

Meet the optical transmission method.

Images