Semiconductor optical modulator

Abstract: A low-loss passive region is provided on one side or both sides of a waveguide-type optical modulator. Thereby, it is possible to obtain a high-speed optical modulator package superior in mechanical strength, thermal characteristic, high-frequency characteristic, and extinction characteristic. Moreover, it is possible to increase the operating speed of an optical fiber transmission equipment by applying the optical modulator package to the optical fiber transmission equipment.

Description: Background of the Invention The present invention relates to a semiconductor optical modulator, an optical modulator package using the semiconductor optical modulator, and an optical fiber transmission equipment. To increase the capacity of an optical fiber transmission equipment, it is necessary to realize higher-speed light modulation. An optical modulator using the electric field effect of semiconductor can operate at a high-speed, high-efficiency light modulation. Therefore, it has been studied how to increase the operating speed of the modulator and to package the modulator in order to apply the modulator to optical fiber transmission equipment. FIGs. 1A to 1C show a conventional electro- absoprtion optical modulator which is one type of semiconductor optical modulators. In FIGs. 1A to 1C, numeral 1 denotes an n-doped semiconductor wafer, 2 denotes an n-doped buffer layer, 3 denotes an undoped light absorption layer, 4 denotes a p-doped cladding layer, 5 denotes a contact layer, 6 denotes an optical waveguide, 7 denotes an insulator, 8 denotes a p electrode, 9 denotes an n electrode, and 10 denotes an anti-reflection coat. In this modulator, light enters the optical waveguide from an element facet and is propagated through the optical waveguide, and thereafter it is emitted from the opposite facet. The optical waveguide has the light absorption layer 3, and light is modulated by applying an electric field to the light absorption layer by use of electrodes and by changing the absorption intensity. By adopting a multiple-quantum well structure in the light absorption layer 3 so as to utilize the electro- absorption effect, an optical modulator with a higher efficiency is obtained. FIGs. 2A and 2B show an optical modulator package using the above semiconductor optical modulator. In FIGs. 2A and 2B, numeral 21 denotes a package body, 22 denotes a polarization holding fiber, 23 denotes a carrier, 24 denotes an unspherical lens, 251 and 252 denotes microstrip lines, 26 denotes an optical modulator, 27 denotes a matching resistor, 281 and 282 represent wire bonds, 29 denotes input/output terminal pins, 30 denotes a rod lens, 31 denotes an electronic cooling device, and 32 denotes a package cover. In the modulator package, optical coupling of the input/output optical fiber 22 and the optical modulator 26 is achieved by the rod lens 30 and unspherical lens 24. The element is mounted on the carrier 32 having a protrusion with a width (W) approximately equal to the modulator length (L) so that light is not scattered between the modulator 26 and the unspherical lens 24. An electrical signal is fed to the modulator through the microstrip line 251 and moreover, the matching resistor 27 for avoiding reflection of the signal is provided at the end of the strip line 251. This type of package is described in, for example, the TECHNICAL REPORT OF IEICE, EMD 93-33, CPM 93-46, OQE 93-67 (1993-08), pp. 55-60. Summary of the Invention The modulation speed of the above optical modulator is limited by the element capacitance. Therefore, to obtain a higher-speed optical modulator, it is necessary to decrease the element capacitance by decreasing the modulator length (L). However, when decreasing the device length (L) of a conventional optical modulator and optical modulator package in order to increase the operation speed, it is necessary to also decrease the width (W) of the protrusion of a carrier in order to avoid scattering of light. However, if the width (W) of the carrier protrusion is decreased, a problem arises that the mechanical strength of the protrusion is decreased or heat conduction becomes low. Moreover, it is necessary to decrease the microstrip line 251 and the matching resistor 27 in size provided on the same protrusion, otherwise a problem brings up that the high frequency characteristics of the strip line and the withstand voltage of the matching resistor are impaired. Furthermore, when the element has a length of 300 µ m or less, the light not coupled with the optical waveguide at the incidence facet of the modulator is propagated through the part above the optical modulator or within the wafer of the optical modulator and coupled with an optical fiber on the output side. The uncoupled light impairs the extinction characteristics of the package or makes the adjustment of the optical system difficult. US patent document US-A 05 359 679 deals with the problem of increasing the cut-off frequency of an optical modulator. Therein, is described a modulator device which has a waveguide region provided at either of two sides of the modulation region to reduce the capacitance of the device and prevent the pn junctions from exposure to air. The p-side electrode and the p-type clad layer are laminated in the modulation region alone, the effective length of the device wherein the capacitance is formed is limited only to the modulation region and hence the capacitance of the device can be reduced by arbitrarily selecting the length of the modulation region. Thus a modulator of a high cut-off frequency can be obtained. Contrary to the known structure of the optical modulator according to US-A 05 359 679, the optical modulator of the present invention does not comprise a single continuous layer as the core layers of the passive and modulation region but the core layers of the passive regions are discontinuous with the core layer of the modulation region. The bandgap energy of the core layer in the passive region is greater than that of the core layer in the modulation region. Therefore the passive regions of the present device allow the incident light to travel with low loss, realising a more efficient optical modulator. It is an object of the present invention to provide a high-speed semiconductor optical modulator suitable for packaging, an optical modulator package using the optical modulator, and an optical fiber transmission equipment. The above object is achieved by the optical modulation device defined in claim 1. Functions of the optical modulator according to the invention will be described below, referring to FIGs. 3A to 3C. In FIGs. 3A to 3C, numeral 41 denotes an n-InP wafer, 42 denotes an n-InAlAs buffer layer, 43 denotes an undoped InGaAs/InAlAs strained multiple- quantum well layer, 44 denotes a p-InAlAs cladding layer, 45 denotes a p+-InGaAs contact layer, 46 denotes an SiO 2 mask, 47 denotes an undoped InP buffer layer, 48 denotes an undoped InGaAsP core layer, 49 denotes an undoped InP cladding layer, 50 denotes an optical waveguide, 51 denotes polyimide, 52 denotes a Cr/Au p-electrode, 53 denotes an AuGeNi n-electrode, and 54 denotes an SiNx anti-reflection coat. In this modulator, the optical waveguide structure comprises a modulation region (length of L 2 ) for modulating light and two passive regions (lengths of L 1 and L 3 ) for coupling the modulation region with the element facets. The band gap energy of the semiconductor constituting the optical waveguide in the passive region is sufficiently larger than the light energy, and the light is propagated in these regions with a low loss. The light entering through a facet is propagated through the passive region, modulated in the modulation region, propagated through the passive region again, and emitted outside. Because the cladding layer 49 in the passive region is formed of undoped or semi-insulating semiconductor, the element capacitance is not increased even when the passive regions are provided. Therefore, in this optical modulator, it is possible to easily provide a high speed device by decreasing the modulation-region length (L 2 ) even though the overall device length (L=L 1 +L 2 +L 3 ) is long. Moreover, in the case of an optical modulation package using this high speed modulator, it is possible to increase the carrier protrusion width (W) almost up to the overall element length (L). Therefore, by using this modulator, it is possible to provide a high-speed optical modulator package superior in the mechanical strength, thermal characteristic, high-frequency characteristic, and extinction characteristic. The device has been described above in which optical waveguides are provided on both sides of a modulation region. However, it should be noted that the same effect can be expected by forming an optical waveguide on only one side of the modulation region. Moreover, though the method for insulating the whole of the cladding layer 49 in the passive region has been described above, completely the same effect can be expected by forming high-resistance regions between the passive regions and the modulation region to electrically isolate the cladding layers in both regions from each other even if the cladding layers in the passive regions are conductive. Brief Description of the Drawings FIG. 1A is a top view of a conventional semiconductor optical modulator, FIG. 1B is a sectional view along the line A-A' of FIG. 1A, and FIG. 1C is a sectional view along the line B-B' of FIG. 1A; FIG. 2A is a top view of a conventional semiconductor optical modulator package (however, the package cover is removed), and FIG. 2B is a sectional view along the line A-A' of FIG. 2A (the light propagation state is also shown); FIG. 3A is a top view of embodiment 1 of the optical modulator of the present invention, FIG. 3B is a sectional view along the line A-A' of FIG. 3A, and FIG. 3C is a sectional view along the line C-C' of FIG. 3A; FIG. 4A is a top view of embodiment 2 of the optical modulator of the present invention, FIG. 4B is a sectional view along the line A-A' of FIG. 4A, and FIG. 4C is a sectional view along the line B-B' of FIG. 4A; FIG. 5A is a top view of embodiment 3 of the optical modulator of the present invention, and FIG. 5B is a sectional view along the line A-A' of FIG. 5A; and FIG. 6 is an illustration showing the optical modulator of the present invention and an optical fiber transmission equipment using the optical modulator. Description of the Preferred Embodiments FIG. 3A is a top view of the optical modulator of embodiment 1, FIG. 3B is a sectional view along the line A-A' of FIG. 3A, and FIG. 3C is a sectional view along the line C-C' of FIG. 3A. The B-B' cross section in FIG. 3A is the same as that in FIG. 1C. Numeral 46 in FIG. 3A denotes an SiO2 mask used for etching and the second crystal growth. The n-In 0.53 Al 0.47 As buffer layer 42 (0.2 µ m), undoped In 0.48 Ga 0.52 As/In 0.60 Al 0.40 As strained multiple- quantum well layer 43 (well layer: 7 nm, barrier layer: 5 nm, number of wells: 10, and band gap wavelength: 1.49 µ m), p-In 0.53 Al 0.47 As cladding layer 44 (2.0 µ m), and p+-In 0.53 Ga 0.47 As contact layer 45 (0.2 µ m) were formed in order on the n-InP wafer 41 by an MBE (Molecular Beam Epitaxy) technique. Then, the MBE growth layer in the region other than the SiO 2 mask 46 was removed by selective wet etching using the SiO 2 mask 46. Then, the undoped InP buffer layer 47 (0.15 µ m), undoped In 0.82 Ga 0.18 As 0.40 P0.60 core layer 48 (0.2 µ m), and undoped InP cladding layer 49 (1.5 µ m) were grown in the region other than the SiO2 mask 46 by MOCVD (Metal Organic Chemical Vapor Deposition). After the SiO 2 mask is removed, mesa-etching up to an InP wafer was performed by dry-etching to form an optical waveguide 50 (waveguide width of 3 µ m). After the element surface was planarized by polyimide 51, the Cr/Au p-electrode 52 was formed. After the AuGeNi n-electrode 53 is formed by vacuum-evaporation, the element is cleaved, and SiNx anti-reflection coats 54 are formed on both facets. The element had a length of 1,000 µ m, the modulation region had a length (L 1 ) of 100 µ m, and the element had a capacitance of 0.3 pF. The optical modulator package shown in FIGs. 2A and 2B similar to a conventional one was fabricated using the thus obtained modulator. In the case of this modulator package, the carrier protrusion width (W) was 1,000 µ m. A semiconductor laser beam of 1.55 µ m was applied to the obtained optical modulator package to measure the modulation characteristics. In this measurement, the incidence polarization was adjusted so that the light in the modulator was propagated in the TE mode. The insertion loss in the state that no voltage was applied to the modulator (light output is ON) was 9 dB and the voltage necessary for 15-dB extinction was 1.5 V. The modulation band width (f 3dB ) of the optical modulator package was 20 GHz. An optical fiber transmission equipment (transmission rate of 10 Gbits/s) shown in FIG. 6 was manufactured using the obtained optical modulator package. In FIG. 6, numeral 90 denotes a 1.55- µ m DFB laser, 91 denotes an optical modulator driving circuit, 92 denotes an optical modulator package, 93 denotes an optical fiber amplifier, and 94 denotes a dispersion-shift optical fiber. The light emitted from the DFB (distributed feedback) laser 90 with a oscillation wavelength of 1.55 µ m was modulated by the optical modulator package 92, amplified by the optical fiber amplifier 93, transmitted through the dispersion-shift optical fiber 94 (100 km), and received by a receiver. The error rate of the optical fiber transmission equipment was measured, and a good value of 10-12 or less was obtained. FIG. 4A is a top view of the optical modulator of embodiment 2, and FIG. 4B is a sectional view along the line A-A' of FIG. 4A. The broken line 46 in FIG. 4A shows an SiO2 mask used for the etching and the second crystal growth. In FIGs. 4A and 4B, numeral 61 denotes an n-InP buffer layer, 62 denotes an undoped In GaAsP absorption layer, 63 denotes a p-InP cladding layer, 64 denotes an Fe-doped semi-insulating InP upper cladding layer, 651 and 652 denote n-InP cap layers, and 66 denotes an Fe-doped semi-insulating InP buried layer. The n-InP buffer layer 61 (0.2 µ m), undoped In 0.63 Ga 0.37 As 0.82 P 0.18 absorption layer 62 (0.2 µ m, band gap wavelength: 1.47 µ m), p-InP cladding layer 63 (2.0 µ m), and p+-In 0.53 Ga 0.47 As contact layer 45 (0.2 µ m) were formed in order on the n-InP wafer 41 by MOCVD. Then, the contact layer 45, cladding layer 63, and absorption layer 62 other than the SiO 2 mask 46 were removed by selective wet etching using the SiO 2 mask 46. Then, the undoped In 0.82 Ga 0.18 As 0.40 P 0.60 core layer 48 (0.2 µ m), Fe-doped semi-insulating InP cladding layer 64 (2.0 µ m), and n-InP cap layer 651 were formed in the region other than the SiO 2 mask 46 by MOCVD (Metal Organic Chemical Vapor Deposition). After the SiO 2 mask 46 was removed, etching is performed up to the InP wafer 41 by wet etching to form the optical waveguide 50 (waveguide width of 2 µ m) and the waveguide was embedded with the Fe-doped InP 652 and n-InP 652. The Cr/Au p-electrode 52 was formed to partly remove the n-InP cap layers in the region other than the electrode. After the AuGeNi n-electrode 53 was vacuum-evaporated, the element was cleaved and the SiNx anti-reflection coats 54 were formed on both facets of the element. The element had an overall length of 1,000 µ m, the modulation region has a length of 150 µ m, and the element had a capacitance of 0.4 pF. An optical modulator package was manufactured by using the above optical modulator similarly to embodiment 1. The carrier protrusion width (W) was 1,000 µ m similarly to embodiment 1. The insertion loss of the obtained modulator was 6 dB, the modulation band was 15 GHz, and the voltage necessary for 15-dB extinction was 3.0 V. An optical fiber transmission equipment like embodiment 1 was fabricated by using the optical modulator, and it was a good transmission equipment with an error rate of 10 -12 or less. FIG. 5A is a top view of the optical modulator of embodiment 3, and FIG. 5B is a sectional view along the line A-A' of FIG. 5A. The broken lines in FIG. 5A show SiO 2 masks 801 and 802 used for crystal growth. In FIG. 5B, numeral 81 denotes an n-InGaAsP lower guiding layer, 82 denotes an InGaAs/InGaAsP multiple- quantum well layer, 83 denotes a p-InGaAsP upper guiding layer, 84 denotes a p-InP cladding layer, and 85 denotes proton implantation. The SiO 2 masks 801 and 802 were provided on an n-InP wafer, and the n-In 0.82 Ga 0.18 As 0.40 P 0.60 lower guiding layer 81 (0,15 µ m), undoped In 0.53 Ga 0.47 As/ In 0.82 Ga 0.18 As 0.40 P 0.60 multiple-quantum well layer 82 (well layer: 4 nm, barrier layer: 7 nm, number of wells: 10, band gap wavelength: 1.45 µ m), and p-In 0.82 Ga 0.18 As 0.40 P 0.60 upper guiding layer 83 (0.15 µ m) were selectively grown in the region other than the SiO 2 masks by MOCVD. In this case, in the modulation region sandwiched by the SiO 2 masks 801 and 802, the growth rate and mixed crystal composition were changed due to the material flowing from the tops of the masks, and the band gap wavelength of a multiple quantum well became larger than those of the passive regions. In the above case, however, the opening widths of the SiO 2 masks were adjusted so that the multiple quantum well band gap wavelength in the modulation region was 1.50 µ m. After the SiO 2 masks 801 and 802 were removed, the p-InP cladding layer 84 (2.0 µ m) and p+-In 0.53 Ga 0.47 As contact layer 45 (0.2 µ m) were formed on the entire surface of the wafer by MOCVD. A resist mask was placed on the light modulation region to partly remove the contact layer other than the light modulation region. Then, protons were implanted into the part between the modulation region and the passive region to insulate the passive-region cladding layer from the modulation-region cladding layer. Mesa-etching was performed up to the InP wafer 41 by dry etching to form the optical waveguide 50 (waveguide width of 3 µ m), and thereafter planarized with polyimide 51 to form the Cr/Au p-electrode 52. After forming the AuGeNi n-electrode 53, the element was cleaved to form the SiNx anti-reflection coats 54 on both facets of the element. The element had an overall length of 800 µ m and the modulation region had a length of 100 µ m. The obtained element had a capacitance of 0.3 pF. An optical module package was manufactured using the above optical modulator in the same manner as of embodiment 1. The carrier width (W) was 800 µ m. The insertion loss of the obtained optical modulator was 6 dB, the modulation band was 18 GHz, and the voltage necessary for 15-

Claims: A semiconductor optical modulator comprising: a semiconductor wafer (41), an optical waveguide (50) which is formed on the semiconductor wafer and comprises a modulation region for modulating light and a passive region for propagating light on one side or both sides of the modulation region, and means (52, 53) for applying an electric field to the modulation region, characterised in that the modulation region includes a core layer (43, 62) and a cladding layer (44, 63), and the passive region includes a core layer (48) whose band gap energy is larger than the band gap energy of the core layer (43, 62) of the modulation region, and the core layer (48) in the passive region is discontinuous with the core layer (43, 62) in the modulation region, The modulator of claim 1, wherein the modulation region of the optical waveguide has a length L 2 of less than 300 µm and the optical waveguide has an overall length of 300 µm or more. The modulator of claim 1 or 2, wherein at least part of a cladding layer (49) in the passive region is made of undoped semiconductor or semi-insulating semiconductor. The modulator of any preceding claim, wherein the cladding layer (49) in the passive region is electrically isolated from the cladding layer in the modulation region. The modulator of any preceding claim, wherein light is modulated by use of the electro-absorption effect of semiconductor. The modulator of any preceding claim, wherein at least part of the optical waveguide comprises a multiple-quantum well (82). The modulator of any preceding claim, wherein at least part of the optical waveguide comprises a strained multiple-quantum well (82). The modulator of any preceding claim, wherein a semiconductor doped with Fe or a semiconductor into which protons are implanted is used. An optical modulator package using the semiconductor optical modulator of any preceding claim. The modulator package of claim 9, wherein a protrusion of a carrier on which the modulator is mounted has a width of 300 µm or more. An optical fiber transmission equipment using the optical modulator package of claim 9 or 10. A method of manufacturing the modulator of any preceding claim, wherein a multilayer structure serving as an optical waveguide in the modulation region is provided on the entire surface of the wafer, at least part of the multilayer structure in the passive region is removed, and a multilayer structure constituting at least part of an optical waveguide in the passive region is formed in the passive region again. A method of manufacturing the modulator of any of claims 1 to 12, wherein at least part of multilayer structures constituting the optical waveguides in the modulation and passive regions are simultaneously formed by selective-area growth with an insulating mask placed on the semiconductor.

Inventors: IDO TATEMI, SANO HIROHISA, IDO, TATEMI, SANO, HIROHISA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: G02F1
IPC codes: G02B6, G02F1