Method for selecting a semiconductor integrated optical modulator/laser light source device

Abstract: The present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section (3) and a laser section (2). The method comprises the following steps. The laser section (2) is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section (2) to show a laser emission when the modulator section (3) is supplied with a pulse signal for allowing the modulator section (3) to show a pulse modulation so as to allow a light emitted directly from a facet of the laser section (2) to show a transmission through a light wavelength dispersion medium within which the light on transmission shows a dispersion in wavelength. An intensity of the light having shown the dispersion in wavelength is measured to find a coefficient of variation in intensity of the light, wherein the variation is due to a wavelength chirping. There is verified the fact that the coefficient of variation in intensity of the light be below a predetermined reference level.

Description: The present invention relates to a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. There would be coming attractive a semiconductor optical modulator integrated laser light source device having an integration of a laser device such as a distributed fed back laser or a distributed reflection laser and a modulator such as an electroabsorption modulator on account of this light source device shows a small chirping when the modulator shows a modulation. The property of the small chirping of the laser device is suitable for a long-distance and large capacitance optical signal transmission. In the semiconductor optical modulator integrated laser light source device, the laser section is driven by a direct current and the modulator section is driven by pulse signals having binary levels wherein when one level is applied onto the modulator section, the modulator performs an attenuation function thereby the laser beam is absorbed in the modulator, resulting in almost no laser emission being obtained. By contrast, another level is applied onto the modulator section, the modulator shows no attenuation function thereby the laser beam pass through in the modulator without any attenuation, resulting in a laser emission being obtained. This semiconductor optical modulator integrated laser light source device is, however, engaged with problems in generation of bit error due to the following reasons. A first reason for generation of the bit error associated with the semiconductor optical modulator integrated laser light source device is concerned with a generation of wavelength chirping which is caused by optical and electrical interference between the laser section and the modulator section in the semiconductor optical modulator integrated laser light source device. The optical and electrical interference between the laser section and the modulator section may often be generated when an isolation resistance between the laser section and the modulator section is small or when the modulator section has a facet showing an insufficient reflective suppression function. A second reason for generation of the bit error associated with the semiconductor optical modulator integrated laser light source device is concerned with a variation in optical power due to an influence of an electrical multiplication reflection which is caused by an impedance mismatching between the modulator and a modulator driver circuit for driving the modulator. A third reason for generation of the bit error associated with the semiconductor optical modulator integrated laser light source device is concerned with an increase of the wavelength chirping of the modulator section due to rising an efficient of the phase modulation of the which is caused by a carrier accumulation. The carrier accumulated is generated by an optical absorption into the modulator section. Therefore, it is required to verify whether the semiconductor optical modulator integrated laser light source device is available to be free from the above described problem with the bit error. In the prior art, the verifying process is accomplished by making, a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. Thus, it has been required to develop a quite novel method for selection thereof free from the above problem. Accordingly, it is an object of the present invention to provide a novel convenient method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section free from any problems as described above. A method in accordance with the preambles of claims 1 and 6 is known from ELECTRONICS LETTERS, vol. 30, no. 11, 26 May 1994 STEVENAGE GB, pages 872-873, T.KATAOKA ET AL. In addition, a method in accordance with the preamble of claim 6 is known from any of the following documents: ELECTRONICS LETTERS, vol. 26, no. 1, 4 January 1990, pages 9/10, SODA ET AL; JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 6, no. 6, 1 June 1988 pages 779-785, SUZUKI M ET AL; or JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. Lt-5, no. 9, September 1987 NEW YORK, US, pages 1277-1285, M. SUZUKI ET AL. The present invention provides a method as defined in claims 1 or 6. Embodiments of the present invention are set out in the dependent claims. The above and other objects, features and advantages of the present invention will be apparent from the following descriptions. The present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission when the modulator section is supplied with a pulse signal for allowing the modulator section to show a pulse modulation so as to allow a light emitted directly from a facet of the laser section to show a transmission through a light wavelength dispersion medium within which the light on transmission shows a dispersion in wavelength. An intensity of the light having shown the dispersion in wavelength is measured to find a coefficient of variation in intensity of the light, wherein the variation is due to a wavelength chirping. There is verified the fact that the coefficient of variation in intensity of the light be below a predetermined reference level. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission when the modulator section is supplied with a binary pulse signal both with first and second voltage levels and with a frequency which is within a practically available range for allowing the modulator section to show a pulse modulation where the modulator section performs no light attenuation function in the first voltage level of the binary pulse signal and the modulator section performs a light attenuation in the second voltage level. Any of first and second lights emitted from facets of the laser section and the modulator section respectively is caused to show spectra with first and second peaks in intensity versus frequency, provided that the first peak appears only in the first voltage level of the binary pulse signal and the second peak appears only in the second voltage level of the binary pulse signal. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission when the modulator section is reversely biased with first and second direct-current voltages respectively corresponding to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section wherein the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. A light emitted from a facet of the laser section is caused to show spectra with first and second peaks in intensity versus frequency, provided that the first peak appears only in the first direct-current voltage and the second peak appears only in the second direct-current voltage. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. There are measured both a first threshold current of the laser section when the modulator section is reversely biased with a first direct-current voltage and a second threshold current of the laser section when the modulator section is reversely biased with a second direct-current voltage. The first and second direct-current voltages respectively correspond to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section wherein the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. There is verified the fact that a difference between the first and second threshold currents be below a predetermined reference value. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. There are measured both a first power of a light emitted from a facet of the laser section supplied with a direct current which is equal to or larger than a threshold current when the modulator section is reversely biased with a first direct-current voltage and a second power of the light emitted from the facet of the laser section supplied with a direct current which is equal to or larger than a threshold current when the modulator section is reversely biased with a second direct-current voltage, wherein the first and second direct-current voltages respectively correspond to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section and further the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. There is verified the fact that a difference between the first and second powers of the light be below a predetermined reference value. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. There are measured both a first power of a light emitted from a facet of the modulator section when a terminal of the modulator section has a ground potential and a second power of the light emitted from the facet of the modulator section when the terminal of the modulator section is electrically floated so that the modulator section is forwardly biased with a built-in-potential generated at a p-n junction in the modulator. There is verified the fact that a difference between the first and second powers of the light be below a predetermined reference value. The present invention also provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission so as to allow a light emitted from the laser section to pass through the modulation section within which the light shows an absorption thereby electron-hole pairs which may act as carriers are generated wherein a terminal of the modulator section is grounded to cause a photo current due to the electron-hole pairs generated. There is verified the fact that the photo current caused be below a predetermined reference value. Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a first embodiment according to the present invention. FIG. 2 is a diagram illustrative of a variation in power penalty of a light having shown a transmission via an optical fiber versus a coefficient of variation in intensity of the light in a first embodiment according to the present invention. FIGS. 3 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a second embodiment according to the present invention. FIG. 4 is a diagram illustrative of a variation in power penalty of a light having been emitted from a facet of a laser section versus a variation in frequency of the light in a second embodiment according to the present invention. FIG. 5 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a third embodiment according to the present invention. FIG. 6 is a diagram illustrative of a variation in frequency of a light having been emitted from a facet of a laser section versus a reverse bias voltage applied onto a modulator section in a third embodiment according to the present invention. FIG. 7 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a fourth embodiment according to the present invention. FIG. 8 is a diagram illustrative of a variation in threshold current of a laser section versus a reverse bias voltage applied onto a modulator section in a fourth embodiment according to the present invention. FIG. 9 is a diagram illustrative of a variation in power penalty of a light emitted from a facet of a laser section versus a coefficient of variation in threshold current of the laser section in a fourth embodiment according to the present invention as well as illustrative of a variation in power penalty of a light emitted from a facet of a laser section versus a coefficient of variation in power of the light in a fifth embodiment according to the present invention FIG. 10 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a fifth embodiment according to the present invention. FIG. 11 is a diagram illustrative of a variation in power of a light emitted from a facet of a laser section versus a reverse bias voltage applied onto a modulator section in a fifth embodiment according to the present invention. FIG. 12 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a sixth embodiment according to the present invention. FIG. 13 is a diagram illustrative of a variation in power of a light emitted from a facet of a laser section versus a direct current applied to a laser section at both times when a terminal of a modulator section is electrically floated and grounded in a sixth embodiment according to the present invention. FIG. 14 is a diagram illustrative of a variation in power penalty of a light emitted from a facet of a laser section versus a coefficient of variation in power of the light between times when a terminal of a modulator section is electrically floated and grounded in a sixth embodiment according to the present invention. FIG. 15 is a view illustrative of a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section in a seventh embodiment according to the present invention. FIG. 16 is a diagram illustrative of a variation in power penalty of a light emitted from a facet of a laser section versus a photo current generated due to an optical absorption of a laser in a modulator section in a seventh embodiment according to the present invention. The first present invention is to provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. This method is to verify whether the laser device is free from a wavelength chirping which is caused by optical and electrical interference between the modulator section and the laser section when an isolation resistance between the modulation section and the laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission. The modulator section is supplied with a pulse signal for allowing the modulator section to show a pulse modulation. As a result, if an isolation resistance between the modulator section and the laser section is insufficient, the laser section receives an influence of the wavelength chirping caused from the modulator section. A light emitted directly from a facet of the laser section shows a transmission through a light wavelength dispersion medium within which the light on transmission shows a dispersion in wavelength. If the wavelength chirping of the light is generated in the laser section, the light wavelength dispersion medium causes the variation in intensity of the light having transmitted via the light wavelength dispersion medium. An intensity of the light having shown the dispersion in wavelength is measured to find a coefficient of variation in intensity of the light, wherein the variation is due to the wavelength chirping. There is verified the fact that the coefficient of variation in intensity of the light be below a predetermined reference level. If the coefficient of variation in intensity of the light be below a predetermined reference level, this means that the isolation resistance between the laser section and the modulator section is sufficiently large thereby it may be evaluated that the integrated modulator/laser light source device is available. If, however, the coefficient of variation in intensity of the light be above the predetermined reference level, this means that the isolation resistance between the laser section and the modulator section is not sufficiently large thereby it may be evaluated that the integrated modulator/laser light source device is unavailable. The predetermined reference level may be set according to a relationship of the coefficient of variation in intensity of light to a power penalty in view of a transmission bit rate as well as a light wavelength dispersion property and a length of the light wavelength dispersion medium, wherein the length is in a direction of the transmission of the light of the light wavelength dispersion medium. The predetermined reference level may be set to allow the power penalty to be equal to or below 1dB. When the wavelength chirping is large, this results in a large coefficient of variation in intensity of the light is also large. If the coefficient of variation in intensity of the light is large, the power penalty is also large. The power penalty has a positive interrelationship to the coefficient of variation in intensity of the light. If the bit rate is large, the reference value is required to be set at a smaller value. The light wavelength dispersion medium may comprise an optical fiber. The optical fiber may have the same length as needed for a practical use. In this case, the predetermined reference level is set according to a relationship of the coefficient of variation in intensity of light to the power penalty in view of a transmission bit rate set as required as well as a light wavelength dispersion property of the optical fiber. In place of use of the optical fiber, the light wavelength dispersion medium may comprise a prism having almost the same light wavelength dispersion property as that of an optical fiber to be practically used. It is preferable that the pulse signal supplied to the optical modulator section comprises a rectangular-shaped pulse wave form. In this case, the pulse signal may preferably have the same rise and decay times, the same amplitude and the same width as those of a pulse signal to be practically used. The pulse signal may preferably have the same rise and decay times, the same amplitude and a different width from those of a pulse signal to be practically used. The optical modulator section may comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The intensity of the light may be measured by use of a photodetector which is commercially available. The second present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an poetical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. This method is to verify whether the laser device is free from a wavelength chirping which is caused by optical and electrical interference between the modulator section and the laser section when an isolation resistance between the modulation section and the laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission when the modulator section is supplied with a binary pulse signal both with first and second voltage levels corresponding to mark and space levels and with a frequency which is within a practically available range for allowing the modulator section to show a pulse modulation where the modulator section performs no light attenuation function in the first voltage level or the mark level of the binary pulse signal and the modulator section performs a light attenuation in the second voltage level or the space level. If the mark level is applied onto the modulator section, a slight laser emission from a facet of the modulator section is generated. If the space level is applied onto the modulator section, a laser emission from a facet of the modulator section is generated. Any of first and second lights emitted from facets of the laser section and the modulator section respectively is caused to show spectra with first and second peaks in intensity versus frequency, provided that the first peak appears only in the first voltage level or the mark level of the binary pulse signal and the second peak appears only in the second voltage level or the space level of the binary pulse signal. The light emitted from the facet of the laser section is free from the attenuation by the modulator section both when the mark level is applied onto the modulator section and when the space level is applied onto the modulator section. By contrast, the light emitted from the facet of the modulator section receives the attenuation only when the space level is applied onto the modulator section thereby the intensity of this light is considerably reduced. On the other hand, there is generated a difference in wavelength of both the first and second lights emitted from the facets of the laser section and the modulator section respectively. The difference in the wavelength of the first lights between when the mark level is applied onto the modulator section and when the space level is applied onto the modulator section is the same as that of the second signal. For that reason, there is available to measure the difference in the wavelength of either the first or second light. The generated difference in the wavelength of the light results in a generation of the difference in frequency of the light. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. The predetermined reference value may be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view of a transmission bit rate as well as a practical transmission distance. In this case, the predetermined reference value may be set to allow the power penalty to be equal to or below 1dB. The spectra may be caused by use of a spectroscope which is commercially available. The practically available range of frequency of the binary pulse signal may be set to correspond to a range in bit rate from 200 kb/s to 200 Mb/s. The binary pulse signal has the same amplitude as that of a pulse signal to be practically used. In this case, the binary pulse signal supplied to the optical modulator section may comprise a rectangular-shaped pulse wave form. The optical modulator section may comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The third present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. This method is to verify whether the laser device is free from a wavelength chirping which is caused by optical and electrical interference between the modulator section and the laser section when an isolation resistance between the modulation section and the laser section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission. The modulator section is reversely biased with first and second direct-current voltages respectively corresponding to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section wherein the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. A light emitted from a facet of the laser section is caused to show spectra with first and second peaks in intensity versus frequency, provided that the first peak appears only in the first direct-current voltage and the second peak appears only in the second direct-current voltage. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. It may optionally be available to add, prior to the verifying step, a further step of compensating for an increment of the difference in frequency between the first and second peaks, wherein the increment is due to an emission light wavelength shift of the laser section and the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The predetermined reference value may advantageously be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view of a transmission bit rate as well as a practical transmission distance. The predetermined reference value may preferable be set to allow the power penalty to be equal to or below 1dB. The predetermined reference value may also be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of the difference in frequency between the first and second peaks, the increment being due to an emission light wavelength shift of the laser section, the shift being caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The spectra may advantageously be caused by use of a spectroscope which is commercially available. The first and second direct-current voltages supplied to the modulator section may practically be set at 0V and -3V respectively. The optical modulator section may optionally comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The fourth present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. This method is to verify whether the laser device is free from a wavelength chirping which is caused by optical and electrical interference between the modulator section and the laser section when an isolation resistance between the modulation section and the laser section. The method comprises the following steps. There are measured both a first threshold current of the laser section when the modulator section is reversely biased with a first direct-current voltage and a second threshold current of the laser section when the modulator section is reversely biased with a second direct-current voltage. The first and second direct-current voltages respectively correspond to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section wherein the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. There is verified the fact that a difference between the first and second threshold currents be below a predetermined reference value. The first and second threshold currents are measured by verifying an emission of the light from the facet of the laser section by use of a photodetector. It may be available to further add the step of compensating, prior to the verifying step, for an increment of the difference between the first and second threshold currents wherein the increment is due to an emission light wavelength shift of the laser section and the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The predetermined reference value may be set according to a relationship of the difference between the first and second threshold currents to a power penalty in view of a transmission bit rate as well as a practical transmission distance. The predetermined reference value is set to allow the power penalty to be equal to or below 1dB. The predetermined reference value may be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of the difference between the first and second threshold currents, the increment being due to an emission light wavelength shift of the laser section, wherein the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The first and second direct-current voltages supplied to the modulator section may practically be set at 0V and -3V respectively. The optical modulator section may comprise either an electroabsorption modulator which may commercially be available or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The fifth present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. This method is to verify whether the laser device is free from a wavelength chirping which is caused by optical and electrical interference between the modulator section and the laser section when an isolation resistance between the modulation section and the laser section. The method comprises the following steps. There are measured both a first power of a light emitted from a facet of the laser section supplied with a direct current which is equal to or larger than a threshold current when the modulator section is reversely biased with a first direct-current voltage and a second power of the light emitted from the facet of the laser section supplied with a direct current which is equal to or larger than a threshold current when the modulator section is reversely biased with a second direct-current voltage, wherein the first and second direct-current voltages respectively correspond to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of the modulator section and further the modulator section performs no light attenuation function in the first direct-current voltage and the modulator section performs a light attenuation in the second direct-current voltage. There is verified the fact that a difference between the first and second powers of the light be below a predetermined reference value. The first and second powers of the light may be measured by use of a photodetector which may commercially be available. It may be available to add a further step of compensating, prior to the verifying step, for an increment of the difference between the first and second powers of the light, the increment being due to an emission light wavelength shift of the laser section, wherein the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The predetermined reference value may be set according to a relationship of the difference between the first and second powers of the light to a power penalty in view of a transmission bit rate as well as a practical transmission distance. In this case, the predetermined reference value may practically be set to allow the power penalty to be equal to or below 1dB. The predetermined reference value may also be set according to a relationship of the difference between the first and second powers of the light to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of the difference between the first and second powers of the light, wherein the increment is due to an emission light wavelength shift of the laser section and the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The first and second direct-current voltages supplied to the modulator section may practically be set at 0V and -3V respectively. The optical modulator section may comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The sixth present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. The method is to verify whether the laser section is free from a variation in output of a light emitted from the laser section wherein the output variation is due to an electrical multiplication reflection caused by an impedance mismatching between the modulator section and a modulator driver circuit could to the modulator for driving the modulator. The method comprises the following steps. There are measured both a first power of a light emitted from a facet of the modulator section when a terminal of the modulator section has a ground potential and a second power of the light emitted from the facet of the modulator section when the terminal of the modulator section is electrically floated so that the modulator section is forwardly biased with a built-in-potential generated at a p-n junction in the modulator. There is verified the fact that a difference between the first and second powers of the light be below a predetermined reference value. The first and second powers of the light may be measured by use of a photodetector which may commercially be available. The predetermined reference value may be set according to a relationship of the difference between the first and second powers of the light to a power penalty in view of a transmission bit rate as well as a practical transmission distance. In this case, the predetermined reference value may practically be set to allow the power penalty to be equal to or below 1dB. In place of floating the terminal of the modulator section, the terminal is applied with such a positive voltage that the p-n junction of the modulator section is forwardly biased with a voltage substantially corresponding to the built-in-potential. The positive voltage is about +0.6V. The optical modulator section may comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. The seventh present invention provides a method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section and a laser section. The selection method is completely free from any undesirable requirement for making a complicated measurement of a property of the transmission bit error rate associated with the semiconductor integrated optical modulator/laser light source device wherein the measurement of the transmission bit error rate property requires conducting an optical signal transmission test under the same conditions as the practical use and further confirming whether or not there exists a bit error for each of an extremely large numbers of bits as signals having transmitted in the test. The method is to verify whether the modulator section is free from a wavelength chirping due to an increased coefficient of a phase modulation of the modulator wherein the increase in the phase modulation coefficient is due to a carrier accumulation in the modulator section by the optical absorption into the modulator section. The method comprises the following steps. The laser section is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission so as to allow a light emitted from the laser section to pass through the modulation section within which the light shows an absorption thereby electron-hole pairs which may act as carriers are generated wherein a terminal of the modulator section is grounded to cause a photo current due to the electron-hole pairs generated. There is verified the fact that the photo current caused be below a predetermined reference value. The photo current is measured by use of an ampere meter which may be commercially available. The predetermined reference value may be set according to a relationship of the photo current to a power penalty in view of a bit error rate as well as a practical transmission distance. The predetermined reference value may practically be set to allow the power penalty to be equal to or below 1dB. The predetermined reference value may optionally be set in further view of a load resistance which is needed to allow the modulator to show a high speed modulation. The optical modulator section may comprise either an electroabsorption modulator or a semiconductor Mach-Zehnder optical modulator. The laser section may comprise a distributed feedback laser. A first embodiment according to the first present invention will be described with reference to FIGS. 1 and 2 in which a method for selecting the semiconductor optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. The laser section 2 is supplied with a direct current DC which is equal to or larger than a threshold current for allowing the laser section to show a laser emission. The modulator section 3 is supplied with a rectangular-shaped pulse signal for allowing the modulator section to show a pulse modulation. The pulse signal is the same as that practically used. As a result, if an isolation resistance between the modulator section 3 and the laser section 2 is insufficient, the laser section 2 receives an influence of the wavelength chirping caused from influences of the interference to the modulator section 3. A light emitted directly from a facet of the laser section is fetched by a photo-detector such as a photo-diode 12 via a an optical fiber 11. The photo-detector then convert the optical signal into an electrical signal which is received by an oscilloscope 13 to display the intensity of the electrical signal to evaluate the intensity of the optical signal having shown the transmission via the optical fiber 11. The optical fiber 11 has the same length as that of the optical fiber practically used. The variation in the intensity of the light is generated by the wavelength chirping which is caused by the rise and decay of the rectangular-shaped pulse signal applied onto the modulator section. FIG. 2 illustrates the power penalty (dB) versus the coefficient of variation in intensity of the light under the condition of a bit rate of 2.5 Gb/s, from which it could be understood that when the power penalty is required to be below 1dB, the coefficient of variation in intensity of the light is required to be below 0.2. Then, the reference value is set at 0.2. There is verified the fact that the coefficient of variation in intensity of the light be below a predetermined reference level of 0.2. If the coefficient of variation in intensity of the light be below a predetermined reference level, this means that the isolation resistance between the laser section and the modulator section is sufficiently large thereby it may be evaluated that the integrated modulator/laser light source device is available. If, however, the coefficient of variation in intensity of the light be above the predetermined reference level, this means that the isolation resistance between the laser section and the modulator section is not sufficiently large thereby it may be evaluated that the integrated modulator/laser light source device is unavailable. A second embodiment according to the first present invention will be described with reference to FIGS. 3 and 4 in which a method for selecting the semiconductor optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. The laser section 2 is supplied with a direct current DC which is equal to or larger than a threshold current for allowing the laser section 2 to show a laser emission when the modulator section is supplied with a binary pulse signal both with first and second voltage levels corresponding to mark and space levels and with a frequency which is within a practically available range for allowing the modulator section to show a pulse modulation. In the pulse modulation, the modulator section 3 performs no light attenuation function in the first voltage level or the mark level of the binary pulse signal and the modulator section performs a light attenuation in the second voltage level or the space level. If the mark level is applied onto the modulator section, a slight laser emission from a facet of the modulator section is generated. If the space level is applied onto the modulator section, a laser emission from a facet of the modulator section is generated. The first light emitted from the facet of the laser section 2 free from the attenuation by the modulator section 3 shows spectra with first and second peaks in intensity versus frequency, provided that the first peak appears only in the first voltage level or the mark level of the binary pulse signal and the second peak appears only in the second voltage level or the space level of the binary pulse signal. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. The predetermined reference value may be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view of a transmission bit rate as well as a practical transmission distance. FIG. 4 illustrates the power penalty versus the difference in frequency between the first and second peaks under the condition of the bit rate set at 2.5Gb/s and different three distance transmissions of 150km, 80km and 40 km. When considering the 80km transmission, the power penalty below 1dB requires the difference in frequency of the light at the first and second intensity peaks to be below 5GHz. The practically available range of frequency of the binary pulse signal may be set to correspond to a range in bit rate from 200 kb/s to 200 Mb/s. The binary pulse signal has the same amplitude as that of a pulse signal to be practically used. In this case, the binary pulse signal supplied to the optical modulator section may comprise a rectangular-shaped pulse wave form. A third embodiment according to the present invention will be described with reference to FIGS. 5 and 6 in which a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. The laser section 2 is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission. The modulator section is reversely biased with -3V or not biased with 0V. The modulator section 3 performs no light attenuation function when 0V is applied onto the modulator section and does perform a light attenuation when -3V is applied onto the modulator section 3. A light emitted from a facet of the laser section 2 is caused to show spectra with first and second peaks in intensity versus frequency, provided that the first peak appears when applying 0V and the second peak appears when applying -3V. There is verified the fact that a difference in frequency between the first and second peaks be below a predetermined reference value. In the verifying step, there is needed compensating for an increment of the difference in frequency between the first and second peaks, wherein the increment is due to an emission light wavelength shift of the laser section and the shift is caused by a heat generation in the modulator section only when reversely biased with -3V. The predetermined reference value may be set according to a relationship of the difference in frequency between the first and second peaks to a power penalty in view not only of a transmission bit rate as well as a practical transmission distance but also the increment of the frequency difference due to the heat generation. FIG. 6 illustrates the difference in frequency of the first and second peaks versus the reverse bias voltage and the increment of the frequency difference due to the heat generation caused by the reverse bias. FIG. 6 is used when making the compensation for the increment of the frequency difference due to the heat generation. There is verified whether the compensated frequency difference be below the predetermined reference value which is determined by use of the relationship of the power penalty to the frequency difference between the first and second peaks under the condition of the bit rate set at 2.5Gb/s and different three distance transmissions of 150km, 80km and 40 km illustrated in FIG. 4. When considering the 80km transmission, the power penalty below 1dB requires the compensated difference in frequency of the light at the first and second intensity peaks to be below 5GHz. A fourth embodiment according to the present invention will be described with reference to FIGS. 7 and 8 in which a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. There are measured both a first threshold current of the laser section 2 when the modulator section is reversely biased with 0V and a second threshold current of the laser section when the modulator section is reversely biased with -3V. The modulator section performs no light attenuation function when applying 0V onto the modulator section 3 and the modulator section 3 performs a light attenuation when applying -3V onto the modulator section 3. There is verified the fact that a difference between the first and second threshold currents be below a predetermined reference value by confirming a laser emission from the laser section 2 by use of a photo-diode 12. In the verifying step, there is made a compensation for an increment of the difference between the first and second threshold currents wherein the increment is due to an emission light wavelength shift of the laser section and the shift is caused by a heat generation in the modulator section only when reversely biased with the second direct current voltage. The predetermined reference value may be set according to a relationship of the difference between the first and second threshold currents to a power penalty in view of a transmission bit rate as well as a practical transmission distance. The predetermined reference value is set to allow the power penalty to be equal to or below 1dB. FIG. 8 illustrates the threshold current Ith (mA) versus the reverse bias voltage applied onto the modulator section 3 as well as an increment of the threshold current difference due to a leak current between the laser section 2 and the modulator section 3. There is needed a compensation for the increment of the threshold current difference due to the leak current between the laser section 2 and the modulator section 3. FIG. 9 illustrates the power penalty versus the coefficient of the variation in threshold current, from which it could be understood that when the power penalty is required to be below 1dB, the coefficient of the variation in threshold current is required to be below 1%. It is verified whether the coefficient of the variation in threshold current calculated from the compensated difference in threshold current be below 1%. A fifth embodiment according to the present invention will be described with reference to FIGS. 10 and 11 in which a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. There are measured both a first threshold current of the laser section 2 when the modulator section 3 is reversely biased with 0V and a second threshold current of the laser section when the modulator section is reversely biased with - 3V. The modulator section 3 performs no light attenuation function when applying 0V onto the modulator section 3 and the modulator section 3 performs a light attenuation when applying -3V onto the modulator section 3. There is verified the fact that a difference between the first and second optical powers be below a predetermined reference value by confirming a laser emission from the laser section 2 by use of a photo-diode 12. The predetermined reference value may be set according to a relationship of the difference between the first and second optical powers to a power penalty in view of a transmission bit rate as well as a practical transmission distance. The predetermined reference value is set to allow the power penalty to be equal to or below 1dB. FIG. 11 illustrates the optical power (mW) versus the reverse bias voltage applied onto the modulator section 3 as well as an increment of the optical power difference due to a leak current between the laser section 2 and the modulator section 3. Referring back to FIG. 9 which illustrates the power penalty versus the coefficient of the variation in optical power, it could be understood that when the power penalty is required to be below 1dB, the coefficient of the variation in optical power is required to be below 15%. It is verified whether the coefficient of the variation in threshold current calculated from the compensated difference in optical power be below 15%. A sixth embodiment according to the present invention will be described with reference to FIGS. 12, 13 and 14 in which a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. There are measured both a first power of a light emitted from a facet of the modulator section 3 when a terminal of the modulator section has a ground potential and a second power of the light emitted from the facet of the modulator section 3 when the terminal of the modulator section 3 is electrically floated so that the modulator section is forwardly biased with a built-in-potential generated at a p-n junction in the modulator. The difference between the first and second powers is found to find the coefficient of the variation in the optical powers. FIG. 14 illustrates the power penalty versus the coefficient of the variation in the power, from which it could be understood that when the power penalty is required to be below 1dB, the coefficient of the variation in the power is required to be below 0.25. There is verified the fact that the coefficient of the variation of the power be below 0.25. A seventh embodiment according to the present invention will be described with reference to FIGS. 15 and 16 in which a novel method for selecting a semiconductor integrated optical modulator/laser light source device comprising a laser section 2 and a modulator section 3 is provided. The laser section 2 is supplied with a direct current which is equal to or larger than a threshold current for allowing the laser section to show a laser emission so as to allow a light emitted from the laser section to pass through the modulation section within which the light shows an absorption thereby electron-hole pairs which may act as carriers are generated wherein a terminal of the modulator section is grounded to cause a photo current due to the electron-hole pairs generated. The modulator section 3 is grounded via an ampere meter 16 for measuring a photo current to evaluate the accumulation state of the carrier generated by the optical absorption in the modulator section 3. FIG. 16 illustrates the power penalty versus the optical current, from which it could be understood that when the power penalty is required to be below 1dB, the photo current is required to be below 5mA. There is verified the fact that the photo current caused be below 5mA. Whereas modifications of the present invention will be apparent to a person having ordinary skill in the art, to which the invention pertains, it is to be understood that embodiments shown and described by way of illustrations are by no means intended to be considered in a limiting sense. Accordingly, it is to be intended to cover by claims all modifications which fall within the scope of the present invention as defined by the claims.

Claims: A method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section (3) and a laser section (2), said method comprising the steps of: supplying said laser section (2) with a direct current which is equal to or larger than a threshold current for allowing said laser section (2) to show a laser emission when said modulator section (3) is supplied with a pulse signal for allowing said modulator section (3) to show a pulse modulation so as to allow a light emitted directly from a facet of said laser section (2) to show a transmission through a light wavelength dispersion medium within which said light on transmission shows a dispersion in wavelength; measuring an intensity of said light having shown said dispersion in wavelength to find a coefficient of variation in intensity of said light, said variation being due to a wavelength chirping; and verifying that said coefficient of variation in intensity of said light be below a predetermined reference level. A method according to claim 1, wherein said method comprises the steps of: supplying said laser section (2) with a direct current which is equal to or larger than a threshold current for allowing said laser section (2) to show a laser emission when said modulator section (3) is supplied with a binary pulse signal with first and second voltage levels and a frequency which is within a practically available range for allowing said modulator section (3) to show a pulse modulation where said modulator section (3) performs no light attenuation function in said first voltage level of said binary pulse signal and said modulator section (3) performs a light attenuation in said second voltage level; causing any of first and second lights emitted from facets of said laser section (2) and said modulator section (3) respectively to show spectra with first and second peaks in intensity versus frequency, provided that said first peak appears only in said first voltage level of said binary pulse signal and said second peak appears only in said second voltage level of said binary pulse signal; verifying that a difference in frequency between said first and second peaks be below a predetermined reference value. A method according to claim 1, wherein said method comprises the steps of: supplying said laser section (2) with a direct current which is equal to or larger than a threshold current for allowing said laser section (2) to show a laser emission when said modulator section (3) is reversely biased with first and second direct-current voltages respectively corresponding to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of said modulator section (3) wherein said modulator section (3) performs no light attenuation function in said first direct-current voltage and said modulator section (3) performs a light attenuation in said second direct-current voltage; causing a light emitted from a facet of said laser section (2) to show spectra with first and second peaks in intensity versus frequency, provided that said first peak appears only in said first direct-current voltage and said second peak appears only in said second direct-current voltage ; and verifying that a difference in frequency between said first and second peaks be below a predetermined reference value. A method according to claim 1, wherein said method comprises the steps of: measuring both a first threshold current of said laser section (2) when said modulator section (3) is reversely biased with a first direct-current voltage and a second threshold current of said laser section (2) when said modulator section (3) is reversely biased with a second direct-current voltage, said first and second direct-current voltages respectively corresponding to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of said modulator section (3) wherein said modulator section (3) performs no light attenuation function in said first direct-current voltage and said modulator section (3) performs a light attenuation in said second direct-current voltage; and verifying that a difference between said first and second threshold currents be below a predetermined reference value. A method according to claim 1, wherein said method comprises the steps of: measuring both a first power of a light emitted from a facet of said laser section (2) supplied with a direct current which is equal to or larger than a threshold current when said modulator section (3) is reversely biased with a first direct-current voltage and a second power of said light emitted from said facet of said laser section (2) supplied with a direct current which is equal to or larger than a threshold current when said modulator section (3) is reversely biased with a second direct-current voltage, said first and second direct-current voltages respectively corresponding to first and second voltage levels of a binary pulse signal to be practically used for a pulse modulation of said modulator section (3) wherein said modulator section (3) performs no light attenuation function in said first direct-current voltage and said modulator section (3) performs a light attenuation in said second direct-current voltage; and verifying that a difference between said first and second powers of said light be below a predetermined reference value. A method for selecting a semiconductor integrated optical modulator/laser light source device comprising a modulator section (3) and a laser section (2), said method comprising the steps of: measuring both a first power of a light emitted from a facet of said modulator section (3) when a terminal of said modulator section (3) has a ground potential and a second power of said light emitted from said facet of said modulator section (3) when said terminal of said modulator section (3) is electrically floated so that said modulator section (3) is forwardly biased with a built-in-potential generated at a p-n junction in said modulator; and verifying that a difference between said first and second powers of said light be below a predetermined reference value. A method according to claim 6, wherein said method comprises the steps of: supplying said laser section (2) with a direct current which is equal to or larger than a threshold current for allowing said laser section (2) to show a laser emission so as to allow a light emitted from said laser section (2) to pass through said modulation section within which said light shows an absorption thereby electron-hole pairs which may act as carriers are generated wherein a terminal of said modulator section (3) is grounded to cause a photo current due to said electron-hole pairs generated; and verifying that said photo current caused be below a predetermined reference value, said photo current preferably being measured by use of an ampere meter. The method as claimed in claim 1, characterized in that said predetermined reference level is set according to a relationship of a coefficient of variation in intensity of light which is preferably measured by a photodetector to a power penalty in view of a transmission bit rate as well as a light wavelength dispersion property and a length of said light wavelength dispersion medium, said length being in a direction of said transmission of said light of said light wavelength dispersion medium, said predetermined reference level preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in claim 1 or 8, characterized in that said light wavelength dispersion medium comprises an optical fiber, said optical fiber preferably having the same length as needed for a practical use, and/or said predetermined reference level preferably being set according to a relationship of a coefficient of variation in intensity of light to a power penalty in view of a transmission bit rate as well as a light wavelength dispersion property of said optical fiber, and/or being set to allow said power penalty to be equal to or below 1dB. The method as claimed in one of claims 1, 8 or 9, characterized in that said light wavelength dispersion medium comprises a prism having almost the same light wavelength dispersion property as that of an optical fiber to be practically used. The method as claimed in one of claims 1, 8, 9 or 10, characterized in that said pulse signal supplied to said optical modulator section (3) comprises a rectangular-shaped pulse wave form, and/or has the same rise and decay times, the same amplitude and the same width as those of a pulse signal to be practically used, or has the same rise and decay times, the same amplitude and a different width from those of a pulse signal to be practically used. The method as claimed in claims 2 or 3, characterized in that said predetermined reference value is set according to a relationship of said difference in frequency between said first and second peaks to a power penalty in view of a transmission bit rate as well as a practical transmission distance and/or said predetermined reference value preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in one of the claims 2, 3 or 12, characterized in that said spectra are caused by use of a spectroscope. The method as claimed in one of the claims 2, 12 or 13, characterized in that said practically available range of frequency of said binary pulse signal corresponds to a range in bit rate from 200 kb/s to 200 Mb/s, said binary pulse signal preferably being the same amplitude as that of a pulse signal to be practically used and further preferably being supplied to said optical modulator section (3) comprises a rectangular-shaped pulse wave form. The method as claimed in claim 10, further comprising the step of: compensating, prior to said verifying step, for an increment of said difference in frequency between said first and second peaks, said increment being due to an emission light wavelength shift of said laser section (2), said shift being caused by a heat generation in said modulator section (3) only when reversely biased with said second direct current voltage. The method as claimed in claim 3, characterized in that said predetermined reference value is set according to a relationship of said difference in frequency between said first and second peaks to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of said difference in frequency between said first and second peaks, said increment being due to an emission light wavelength shift of said laser section (2), said shift being caused by a heat generation in said modulator section (3) only when reversely biased with said second direct current voltage. The method as claimed in claim 3, 4 or 5, characterized in that said first and second direct-current voltages supplied to said modulator section (3) are 0V and -3V respectively. The method as claimed in claim 4, 5 or 6, characterized in that said first and second threshold currents are measured by verifying an emission of said light from said facet of said laser section (2) by use of a photodetector method, or said first and second powers of said light are measured by use of a photodetector, said method preferably comprising the step of: compensating, prior to said verifying step, for an increment of said difference between said first and second threshold currents, said increment being due to an emission light wavelength shift of said laser section (2), said shift being caused by a heat generation in said modulator section (3) only when reversely biased with said second direct current voltage. The method as claimed in claim 4, characterized in that said predetermined reference value is set according to a relationship of said difference between said first and second threshold currents to a power penalty in view of a transmission bit rate as well as a practical transmission distance, said predetermined reference value preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in claim 4, characterized in that said predetermined reference value is set according to a relationship of said difference in frequency between said first and second peaks to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of said difference between said first and second threshold currents, said increment being due to an emission light wavelength shift of said laser section (2), said shift being caused by a heat generation in said modulator section (3) only when reversely biased with said second direct current voltage. The method as claimed in claim 18, characterized in that said predetermined reference value is set according to a relationship of said difference between said first and second powers of said light to a power penalty in view of a transmission bit rate as well as a practical transmission distance, said predetermined reference preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in claim 5, characterized in that said predetermined reference value is set according to a relationship of said difference between said first and second powers of said light to a power penalty in view of a transmission bit rate and a practical transmission distance as well as an increment of said difference between said first and second powers of said light, said increment being due to an emission light wavelength shift of said laser section (2), said shift being caused by a heat generation in said modulator section (3) only when reversely biased with said second direct current voltage. The method as claimed in claim 6, characterized in that said predetermined reference value is set according to a relationship of said difference between said first and second powers of said light to a power penalty in view of a transmission bit rate as well as a practical transmission distance, said predetermined reference value preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in claim 6, characterized in that in place of floating said terminal of said modulator section (3), said terminal is applied with such a positive voltage that said p-n junction of said modulator section (3) is forwardly biased with a voltage substantially corresponding to said built-in-potential, said positive voltage is preverably being +0.6V. The method as claimed in claim 7, characterized in that said predetermined reference value is set according to a relationship of said photo current to a power penalty in view of a bit error rate as well as a practical transmission distance, said predetermined reference value preferably being set to allow said power penalty to be equal to or below 1dB. The method as claimed in claim 25, characterized in that said predetermined reference value is set in further view of a load resistance which is needed to allow said modulator to show a high speed modulation. The method as claimed in one of the claims 1 to 7, characterized in that said optical modulator section (3) comprises an electroabsorption modulator and/or comprises a semiconductor Mach-Zehnder optical modulator. The method as claimed in one of the claims 1 to 7, characterized in that said laser section (2) comprises a distributed feedback laser.

Inventors: YAMAGUCHI MASAYUKI, YAMAGUCHI, MASAYUKI, YAMAGUCHI, MASAYUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H01S3
IPC codes: G01M11, G02F1, H01S5