Abstract:
A method to align an optical device optically with an interference device is disclosed. The method includes steps of: selecting one of arm waveguides, biasing rest of arm waveguides to cause optical absorption thereat, and aligning the optical device optically with the selected arm waveguide.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method to assemble optical device optically with an interfering device. 
         [0003]    2. Related Background Arts 
         [0004]    An optical device combining a semiconductor laser diode (hereafter denoted as LD) with an optically interfering device has been known. When another optical device couples optically with the interfering device, in particular, the other optical devices is optically aligned with the interfering device, light output from the LD and processed in the interfering device is practically used. 
         [0005]    The interfering device in the optical output power thereof depends on not only optical coupling losses but an interference status between phases of two optical beams propagating therein. Accordingly, even when other optical devices are assembled with the interfering device such that the optical output power thereof is set within a preset range, the optical output power in practical usage of the device sometimes deviates from the standard. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the present application relates to a method to assemble an optical module implemented with an interfering device with an optical fiber such that an optical power of light output from the interfering device and entering the optical fiber is set within a preset range. The method comprises steps of: (1) selecting one of arm waveguides formed in the interfering device, the selected one arm waveguide coupling with the optical fiber; (2) biasing rest of arm waveguides not selected in a previous step, the biased arm waveguides causing optical absorption thereat; and (3) aligning the optical fiber with the selected arm waveguide. The arm waveguides not selected in the first step substantially fully absorbs light propagating therein so as not to cause the interference with light propagating in the selected arm waveguides. Accordingly, the light output from the interfering device causes no power fluctuation depending on the phase difference between light each propagating within selected and unselected arm waveguides. 
         [0007]    The interfering device may be an optical modulator of the Mach-Zender type which has an input coupler, an output coupler, and two arm waveguides coupling the input coupler with the output coupler. The input and output couplers may be a type of 2×2 multimode interference (MMI) coupler. Moreover, the Mach-Zender modulator of the embodiment may be formed on a semiconductor substrate, typically, made of InP. The Mach-Zender modulator of the embodiment may receive light in the input coupler thereof emitted from an LD, preferably, a wavelength tunable LD whose emission wavelength is variable. The absorbance of the unselected arm waveguide is preferable greater than 20 dB by receiving the reverse bias. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0009]      FIG. 1A  is a plan view of an optical module according to an embodiment of the invention, and  FIG. 1B  schematically shows the optical module assembled with a coupling unit and an optical fiber; 
           [0010]      FIG. 2A  schematically shows the coupling unit, and  FIG. 2B  is an exploded view of the coupling unit, 
           [0011]      FIG. 3A  is a plan view of a tunable LD, and  FIG. 3B  shows a cross section of the LD taken along the line α-α appeared in  FIG. 3A ; 
           [0012]      FIG. 4A  is a plan view of an interfering device,  FIG. 4B  shows a cross section of the interfering device taken along the line β-β appeared in  FIG. 4A , and  FIG. 4C  also shows a cross section taken along the line γ-γ appeared in  FIG. 4A ; 
           [0013]      FIG. 5  shows behaviors of optical power of each of arm waveguides and a ratio when one of them causes optical loss; and 
           [0014]      FIG. 6A  shows a relation of optical power output from the interfering device against reverse bias applied to one of arm waveguides, and  FIG. 6B  shows a relation of optical power of light propagating in an arm waveguide to which the reverse bias is applied. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Next, some preferred embodiments according to the present invention will be described as referring to drawings. In the description of the drawings, the numerals or symbols same or similar to each other will refer to the elements same or similar to each other without overlapping explanations. 
         [0016]      FIG. 1A  is a plan view showing an optical module  100  to be assembled with an optical fiber by a method according to one embodiment of the invention, and  FIG. 1B  is also a plan view of the optical module  100  assembled with the optical finer  500  via a coupling unit  400 . The optical module  100  according to the embodiment, as shown in  FIG. 1A , includes a package  110 , an optically active device  120 , some optically passive devices, namely, the first lens  130  and the second lens  140 , and a plurality of electrical interconnections  150 . 
         [0017]    The optically active device  120  integrates an LD with an interfering device. The present embodiment provides a wavelength tunable LD as the LD, while, the interfering device is a type of the Mach-Zender device. 
         [0018]    The first lens  130  optically couples one of the first output port  37   a  and the second output port  37   b  of the interfering device  120 , which are shown in  FIG. 4 , with the optical fiber  500  via the second lens  140  and the coupling unit  400 . The first lens  130  is installed within the package  110  after it is mounted on a sub-mount on which the optically active device  120  is also mounted. 
         [0019]    The second lens  140  is rigidly assembled with one of sides of the package  110 . The interconnections  150  are bonded with electrodes of the optically active device  120  with respective bonding wires to provide electrical signals and electrical power to operate the optically active device  120 . Thus, the light output from the LD is modulated by the interfering device, and only one of the output ports,  37   a  and  37   b , emits the light to be provided externally via the first and second lenses,  130  and  140 . 
         [0020]    As shown in  FIG. 1B , the optical module  100  couples with the optical fiber  500  via the coupling unit  400 .  FIG. 2A  illustrates details of the coupling unit  400 , while,  FIG. 2B  is an exploded view of the coupling unit  400 . 
         [0021]    The coupling unit  400  includes a joint sleeve  410 , a receptacle  420  with a ferrule  420   a , and an optical fiber  500  whose end provides another ferrule  430 . The joint sleeve  410  and the receptacle  420  have a cylindrical shape with a center axis coincident with an optical axis of the optical fiber  500 . The joint sleeve  410  provides a flange  410   a  in a root portion thereof, where the flange  410   a  is fixed to the side of the package  110 . Inserting the receptacle  420  in the ferrule  420   a  thereof into a bore  410   b  of the joint sleeve  410 , and adjusting an inserting depth thereof, the optical alignment along the optical axis is carried out, which is often called as the Z-alignment. 
         [0022]    In the present optical module, the Z-alignment is performed by activating the LD  200  practically to guide the light emitted therefrom into the optical fiber  500  and adjust the insertion depth of the receptacle  420  into the joint sleeve  410  such that the optical power of the light output from another end of the optical fiber  500  becomes a preset condition. Thus, the coupling unit  400  is optically aligned with the optical module  100 . In the Z-alignment, the optical power output from the optically active device  120  is necessary to be a preset value independent of optical status of the device  120 . However, as described below, some optical devices, in particular, the interfering device  300  like the present embodiment varies the magnitude of the optical output even when externally set conditions are invariant for devices. 
         [0023]    Next, the optically active device  120  will be described in detail.  FIG. 3A  is a plan view of the LD  200 , while,  FIG. 3B  shows a cross section taken along the line α-α, namely, the optical axis thereof. The LD  200  includes an SOA (Semiconductor Optical Amplifier) region D, an SG-DFB (Sampled Grating Distributed Feedback) region A, a CSG-DBR (Chirped Sampled Grating Distributed Reflector) region B, and an OA (optical absorption) region C, where each of regions are serially arranged in this order along the optical axis. 
         [0024]    The SG-DFB region A stacks, on the semiconductor substrate  1 , a lower cladding layer  2 , an active layer  3 , an upper cladding layer  6 , a contact layer  7 , and an electrode  8 . The CSG-DBR region B also stacks, on the semiconductor substrate  1 , the lower cladding layer  2 , a waveguide layer  4 , the upper cladding layer  6 , an insulating film  9 , and a plurality of micro heaters  10 . Each of micro heaters  10  accompanies with a supply electrode  11  for supplying electrical power thereto and a ground electrode  12 . The OA region C also stacks on the semiconductor substrate  1  the lower cladding layer  2 , an absorption layer  5 , the upper cladding layer  6 , a contact layer  13  and an electrode  14 . The SOA region D stacks on the semiconductor substrate  1  the lower cladding layer  2 , an amplifying layer  19 , the upper cladding layer  6 , a contact layer  20 , and an electrode  21 . In the present embodiment, the lower cladding layer  2  is made of n-type semiconductor material, while, the upper cladding layer  6  and contact layers,  7 ,  13 , and  20  are made of p-type semiconductor materials; but the LD  200  may provide the reverse polarity, that is, the p-type lower cladding layer and the n-type upper cladding layer and the contact layer. Further, the insulating film  9  is also put between the electrode  8  and  21  to operate the SG-DFB region A and the SOA region D independently. 
         [0025]    In the LD  200  thus configured, each of regions, A to D, provide the common substrate  1 , the lower cladding layer  2  and the upper cladding layer  6 ; that is, those layers of the semiconductor substrate  1 , the lower cladding layer  2 , and the upper cladding layer  6  are electrically un-isolated. Moreover, the top level of the lower cladding layer  2  is even in those regions, A to D; that is, the active layer  3  in the SG-DFB region A, the waveguide layer  4  in the CSG-DBR region B, the optical absorption layer  5  in the OA region C, and the amplifying layer  19  in the SOA region D are formed on a substantially flat surface of the lower cladding layer  2 . 
         [0026]    The SG-DFB region A and the CSG-DBR region B provide within the lower cladding layer  2  a plurality of grating regions  18  each including corrugations. Each of the grating regions  18  includes semiconductor materials whose refractive index is different from that of the lower cladding layer  2 . For instance, the grating region may be made of Ga 0.22 In 0.78 As 0.47 P 0.53  when the lower cladding layer  2  is made of InP. One grating region  18  and a region neighbor to this one grating region  18  constitute a segment. 
         [0027]    The CSG-DBR region B includes at least two types of segments each having a specific optical length different from others to show reflectance characteristics different from others; while, the segments in the SG-DFB region A have a length substantially equal to each other. Then, the SG-DFB region A has an optical gain spectrum with a plurality of gain peaks, while, the CSG-DBR region B shows a reflectance spectrum with a plurality of reflectance peaks. The LD  200  may emit light with an emission wavelength at which one of the gain peaks in the SG-DFB region A and one of the reflectance peaks in the CSG-DBR region B coincide. 
         [0028]    In the present embodiment of the LD  200 , the semiconductor substrate  1  is made of n-type InP, the lower cladding layer  2  is made of n-type InP, and the upper cladding layer  6  is made of p-type InP. The lower and upper cladding layers,  2  and  6 , confine light within the active layer  3 , the optical waveguide layer  4 , the absorption layer  5 , and the amplifying layer  19 . 
         [0029]    The active layer  3  is made of material showing an optical gain, for instance, the active layer  3  has a multi-quantum well (MQW) structure comprising alternately arranged well layers of Ga 0.32 In 0.68 As 0.92 P 0.08  each having a thickness of 5 nm and barrier layers of Ga 0.22 In 0.7 As 0.47 P 0.53  each having a thickness of 10 nm. The waveguide layer  4  is made of bulk material of, for instance, Ga 0.22 In 0.78 As 0.47 P 0.53 . 
         [0030]    The absorption layer  5  includes a material having the absorption coefficient at the emission wavelength of the LD  200 . Specifically, the material for the absorption layer  5  has the bandgap wavelength longer than the emission wavelength of the LD  200 , preferably, longer than the longest emission wavelength of the LD  200 . The absorption layer  5  in the present embodiment has the MQW structure including alternately arranged well layers of Ga 0.47 In 0.53 As each having a thickness of 5 nm and barrier layers of Ga 0.28 In 0.72 As 0.61 P 0.39  each having a thickness of 10 nm. In a modification, the absorption layer  5  may be a bulk material of Ga 0.46 In 0.54 As 0.98 P 0.02 . In another modification, the absorption layer  5  may have a structure same with that of the active layer  3 . In this case, the absorption layer  5  may be concurrently formed with the active layer  3 . 
         [0031]    The amplifying layer  19  shows an optical gain by the current injection through the electrode  21 . The amplifying layer  19  in the present embodiment also has the MQW structure including alternately arranged well layers of Ga 0.35 In 0.65 As 0.99 P 0.01  each having a thickness of 5 nm and barrier layers of Ga 0.15 In 0.85 As 0.32 P 0.68  each having a thickness of 10 nm. The amplifying layer  19  may also have the bulk structure of Ga 0.44 In 0.56 As 0.95 P 0.05 . In a modification, the amplifying layer  19  has the same arrangement with the active layer  3 . In this case, the amplifying layer may be concurrently formed with the active layer  3 . 
         [0032]    The contact layers,  7 ,  13 , and  20 , are p-type Ga 0.47 In 0.53 As in the embodiment. The insulating film  9  is made of inorganic material, for instance, SiN, SiO 2  and so on. The micro heaters  10  are formed by a metal thin film made of, for instance, NiCr. The present LD  200  shown in  FIG. 2B  provides a micro heater  10  extending in a plurality of segments, that is, the micro heater  10  is provided in a region including a several grating regions  18  and a space between the grating regions  18 . 
         [0033]    Electrodes,  8 ,  14 , and  21  for respective regions, A, C and D, and those  11  and  12  for the micro heaters  10  are made of metal stacks including gold (Au) in the top thereof. Additionally, the back electrode  15  in the back surface of the substrate  1  is made of also metal stack including Au in the top thereof and extends in all regions of the SG-DFB region A, the CSG-DBR region B, the OA region C, and the SOA region D. The LD  200  provides an anti-reflecting film  16  in the facet of the SOA region D, while, a high-reflecting film  17  in the face of the OA region (C). 
         [0034]    Next, the interfering device  300  will be described.  FIG. 4A  is a plan view of the interfering device  300 , while,  FIGS. 4B and 4C  show cross sections each taken along the line β-β and γ-γ indicated in  FIG. 4A . The interfering device  300  includes a plurality of waveguides each having a mesa structure shown in  FIGS. 4B and 4C  and formed on a semiconductor substrate  41 . 
         [0035]    The mesa waveguide, as shown in  FIGS. 4B and 4C , includes on the semiconductor substrate  41  a lower cladding layer  42   a , a core layer  43 , and an upper cladding layer  42   b  stacked in this order on the substrate  41 . An insulating film  44  and another insulating film  45  cover the surface of the semiconductor substrate  41 , sides of the mesa waveguides, and the top of the mesa waveguide. The lower insulating film  44  has a function of the passivation layer to passivate the surface of the semiconductors. 
         [0036]    The semiconductor substrate  41  is made of InP in the present embodiment. The lower cladding layer  42   a  and the upper cladding layer  42   b  are also made of InP. The core layer  43  is made of semiconductor material whose bandgap energy is smaller than that of the cladding layers,  42   a  or  42   b . For instance, a bulk InGaAsP, an AlGaInAsP with the MQW structure, and so on, are applicable to the core layer  43 . The light propagating in the core layer  43  is confined by the lower and upper cladding layers,  42   a  and  42   b . The lower insulating film  44  is made of InP in the embodiment shown in  FIGS. 4A to 4C , while, the upper insulating film  45  is made of inorganic material, typically, silicon nitride (SiN). 
         [0037]    Referring to  FIG. 4A , the interfering device  300  provides an input waveguide  32   a  connected to the input port  31   a  and an additional waveguide  32   b  terminated within the substrate at an end  31   b  thereof. Two waveguides,  32   a  and  32   b , are coupled and branched to two arm waveguides,  34   a  and  34   b , by the input coupler  33 . The present embodiment of the interfering device  300  provides a 2×2 MMI coupler (Multi Mode Interference) for the input coupler  33 . The LD  200  in the facet of the SOA region D with the anti-reflecting film  16  optically couples with the input port  31   a , that is, the light emitted from the SOA region D of the LD  200  enters the interfering device  300  from the input portion  31   a  thereof. 
         [0038]    One of the arm waveguides  34   a  couples with the other of the arm waveguides  34   b  and branches into two output waveguides,  36   a  and  36   b , at another coupler  35 , namely, an output coupler  35 . Virtually dividing the interfering device  300  along the line extending the axis of the primary waveguide  32   a , one of the output waveguides  36   a , and the output port  37   a  terminating the waveguide  36   a  locate in a side of one of the arm waveguides  34   b ; while, the other of the output waveguides  36   b  and the output port  37   b  locate in the side including the other arm waveguide  34   a . The present embodiment of the interfering device  300  provides a 2×2 MMI coupler as the output coupler  35 . 
         [0039]    Each of the arm waveguides,  34   a  and  34   b , provides an electrode  46  for controlling a phase of light propagating therethrough, and another electrode  47  for modulating the light. The former electrode  46  for controlling the phase positions close to the input port  31   a  is electrically isolated from the other electrode  47  for modulating light, but the arrangement of the electrodes is not restricted to those described above. 
         [0040]    Referring to  FIG. 4C , the modulating electrode  47  positions on the upper cladding layer  42   b  via the contact layer  49 . The contact layer  49  of the present embodiment is made of InGaAs. The insulating films,  44  and  45 , are removed in a region between the upper cladding layer  42   b  and the contact layer  49 . Specifically, the contact layer  49  comes in directly contact to a portion of the upper cladding layer  42   b , and a region exposed from the contact layer  49  in the top of the mesa waveguide is covered by the upper insulating film  45 . The electrode  47  covers the top of the contact layer  49  and a portion of the upper insulating film  45  around the contact layer  49 . Two electrodes include a metal stack including Au as those provided in the LD  200 . 
         [0041]    Supplying a bias voltage to the electrodes,  46  and  47 , the refractive index of the core layer  43  in respective arm waveguides,  34   a  and  34   b , changes depending on the bias, which modifies the phase of the light propagating therethrough. When the interfering device  300  is used as an optical modulator, two signals complementary to each other are applied to respective electrodes  47 , while, two DC biases are applied to the other electrodes  46  to adjust the phase of the light each propagating within the arm waveguides,  34   a  and  34   b . That is, the DC bias applied to the electrodes  46  compensates the phase offset of the light. 
         [0042]    The light entering from the input power  31   a  is divided by the input coupler  33  into two beams each propagating in the arm waveguide  34   a  and the other arm waveguide  34   b , and having a magnitude comparable to each other. These two beams interfere in the output coupler  35  depending on the phase thereof and further divided into two beams each propagating in the output waveguides,  36   a  and  36   b.    
         [0043]    An optical length of the arm waveguide  34   a  and that of the other arm waveguide  34   b  are usually designed to be substantially equal to each other. However, depending on the fabrication process thereof and some other reasons, the phase difference between two optical beams each propagating in respective arm waveguides,  34   a  and  34   b , occasionally scatters from 0 to ±π due to the practical difference of the optical length of respective arm waveguides,  34   a  and  34   b , and the distribution of the light in respective output waveguides,  36   a  and  36   b , depends on this phase difference. Thus, even when the bias conditions applied to respective arm waveguides,  34   a  and  34   b , are equal to each other, conditions between two extrema, one of which is the light fully output from the output port  37   a  and the other is the light fully output from the other output port  37   b , possibly appears. The optical alignment of optical devices with the interfering device  300  using only one of the output ports,  37   a  and  37   b , is impracticable without any adjustment for the phase of the light propagating in the arm waveguides,  34   a  and  34   b . Practically, an adjustment is carried out to even the optical output from the output port  37   a  and that from the other output port  37   b , which is often called as the cross point tuning. However, the cross point tuning is necessary to adjust various conditions of the interfering device  300 . Moreover, these conditions to be adjusted are widely scattered in device to device. The present embodiment provides a nonspecific method to align optical devices with the interfering device  300 . 
         [0044]    The energy bandgap of a semiconductor material depends on the bias applied thereto. Specifically, what is called, the Franz-Keldish effect shifts the fundamental absorption edge of a semiconductor material to a longer wavelength side, namely, to a smaller bandgap energy. Accordingly, one of the arm waveguides,  34   a  and  34   b , is biased to shift the wavelength corresponding to the bandgap energy to a longer side, then, the optical absorption thereat increases to eliminate the interference between two optical beams at the output coupler  35 . In the present embodiment, only one of the arm waveguides,  34   a  or  34   b , transmits the light, where to the light propagating in the other of the arm waveguide is substantially fully absorbed by applying an enough bias thereto, and only one of the output waveguides,  36   a  or  36   b , or one of the output ports,  37   a  or  37   b , outputs the light whose magnitude correlates with the magnitude of the light entering the interfering device  300  at the input port  31   a , where the magnitude of the optical output becomes about a quarter (¼) of that of the optical input. Thus, a universal optical condition independent of internal conditions of the interfering device  300  may be achieved to couple the light output therefrom with the optical fiber  500  by the preset magnitude. 
         [0045]    Although optical losses caused by the input coupler  33 , the output coupler  35 , the arm waveguides,  34   a  and  34   b , and so on affect the optical power output from the port  37   a  or from the other port  37   b . However, such an optical loss may be small enough compared with the interfering effect, and may be ignorable. 
         [0046]    The bias condition to realize the full absorption in the arm waveguides,  34   a  and  34   b , depends on semiconductor materials of the interfering device  300 , which equivalently means that the bias condition is substantially invariant in device to device. Thus, the cross point tuning is no longer unnecessary for the interfering device  300 . 
         [0047]    When the optical absorption in one of the arm waveguides is insufficient to bring the full absorption, the optical output from the interfering device  300  sometimes fluctuates depending on the interfering condition. The present embodiment is necessary to suppress the fluctuation in the optical output within a range allowable in the output from the optical fiber  500 . In an example, when the output fluctuation observed in the output from the optical fiber  500  is necessary to be less than ±1 dB, one of the arm waveguides,  34   a  or  34   b , is necessary to cause the absorption equal to or greater than 20 dB. On the other hand, when the arm waveguide is set in the absorption thereof about 30 dB, the output fluctuation from the optical fiber  500  may be set within ±0.3 dB. 
         [0048]      FIG. 5  shows results for the optical power of respective arm waveguides,  34   a  and  34   b , and a power ratio when one of arm waveguides, where the second arm waveguide  34   b  is selected, causes optical loss by some reasons. In  FIG. 5 , the power ratio is defined by a power when the light propagating in respective arm waveguides,  34   a  and  34   b , is in the state of the in-phase (0°) against a power when the light in respective arm waveguides,  34   a  and  34   b , is in the state of the out-phase (180°). 
         [0049]    When the second arm waveguide  34   b  has no loss, that is, the optical power output from respective arm waveguides,  34   a  and  34   b , are equal to each other, the power ratio becomes infinite because no light is output when the light in the second arm waveguide  34   b  is in the out-phase state. However, a substantial difference in the optical loss due to some reasons, such as, a loss difference in the input and output couplers,  33  and  35 , those causes in respective arm waveguides,  34   a  and  34   b , is unavoidable in a practical device.  FIG. 5  summarizes such an optical loss up to 10 dB is caused in the second arm waveguide  34   b . As shown in  FIG. 5 , when the optical loss in the arm waveguide  34   b  increases, the power ratio due to the phase difference between two arm waveguides,  34   a  and  34   b , becomes small. As described, when no optical loss is caused in the second arm waveguide  34   b , the power ratio is infinite, but it becomes around 5 dB when the optical loss of 10 dB is caused in the second arm waveguide  34   b . Further optical loss is added in the arm waveguide  34   b  by reversely biasing, the power ratio may be further decreased and the power fluctuation by the phase difference between light in respective arm waveguides,  34   a  and  34   b , becomes substantially ignorable. 
         [0050]      FIG. 6A  shows the optical power output from the interfering device  300  when one of arm waveguides,  34   a  or  34   b , is reversely biased. Because the other of arm waveguides,  34   b  or  34   a , is left unbiased, the light output from the interfering device  300  depends on the phase difference of light propagating in respective arm waveguides,  34   a  and  34   b . As shown in  FIG. 6A , biasing one of waveguides,  34   a  or  34   b , not only the optical power output from the device  300  decreases but the power fluctuation thereof decreases. The reverse bias of −10 V or greater decreases the output power by about 6 dB from a state of no reverse bias but stabilizes the power fluctuation thereof, which means that the arm waveguide which is reversely biased fully absorbs the light propagating therein. 
         [0051]      FIG. 6B  shows the relation of the optical power against the reverse bias applied to the arm waveguide. A reverse bias about −10 V to the arm waveguide causes the absorption of the light propagating therein exceeding −30 dB ( 1/1000). Thus, such a reverse bias substantially fully absorbs the light. 
         [0052]    Next, procedures to align optical devices optically with the interfering device  300  will be described. The procedure first sets the optical module  100  on a stage prepared in an aligning apparatus, where the stage is preferable to have a function to vary a temperature of the module placed thereon. Then, the aligning apparatus supplies a reverse bias to one of the arm waveguides,  34   a  or  34   b , of the interfering device  300  to cause the optical absorption thereat by about 20 dB or more. The reverse bias is provided via the electrode  46  for adjusting the phase, that ♭for modulating light, or both. 
         [0053]    Then, the LD  200  is practically activated by injecting current via the electrode  8  provided on the SG-DFB region A. Concurrently with the current injection, the micro heaters  10  are provided with electrical power to set temperatures of the waveguide layer  4  in preset conditions thereof. Furthermore, the temperature of the LD  200  is set in a preset condition by, for instance, a thermo-electric cooler (TEC) installed in the package  110  of the module  100 . Then the LD  200  emits light with an emission wavelength determined by the gain spectrum in the SG-DFB region A and the reflection spectrum in the CSG-DBR region B. 
         [0054]    Finally, as monitoring the optical power emitted from the end of the optical fiber  500 , the Z-alignment between the J-sleeve  410  and the optical receptacle  420  is carried out such that the monitored optical power is within a preset range. The procedure of the Z-alignment sometimes includes an alignment of the coupling unit  400  with the optical module  100  in a plane perpendicular to the optical axis by sliding the flange  410   a  of the J-sleeve  410  on the outer surface of the side of the package  110 . Thus, the optical module  100  with the coupling unit  400  is completed. 
         [0055]    In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. 
         [0056]    For instance, the procedure of the Z-alignment described above fixes the optical module  100  on the stage and the coupling unit  400  is aligned with the optical module  100 . A complementary procedure, where the optical module  100  is aligned optically with the fixed coupling unit  400 , is possible. 
         [0057]    In the embodiment described above, the LD  200  and the interfering device  300  are integrated on the common semiconductor substrate  1 . However, the LD  200  and the interfering device  300  may be separately formed and assembled on a carrier common to two devices, or enclosed in an independent package. Even in such an arrangement, the method of the embodiment of the invention to align optical devices optically with the interfering device  300  is applicable. 
         [0058]    The embodiment thus described concentrates on an arrangement of the interfering device  300  having two arm waveguides. However, the method according to the invention is applicable to other arrangements of the interfering device with three or more arm waveguides. Specifically, all arm waveguides except for the selected one arm waveguide are biased to show enough optical absorption. The Z-alignment with the interfering device may be performed by the light output therefrom without performing the cross-point tuning. 
         [0059]    The embodiment above concentrates on the mach-Zender device as the interfering device. However, the subjects of the present invention are not restricted to those devices. Any devices having the input coupler, a plurality of arm waveguides, and an output coupler are subject to the present invention. For instance, the method of the invention is applicable to the optical frequency doubler. The coupler described above has the arrangement of 2×2 MMI coupler; however, other arrangements of the coupler, such as 1×2 (2×1) MMI, or the directional coupler are applicable. Accordingly, the present specification and figures are to be regarded as illustrative rather than restrictive.