Laser module

Provided is a fiber grating laser for optical communication which can be used as a light source regardless of the occurrence of mode hopping. The laser module comprises a semiconductor optical amplifier device, an optical waveguide such as an optical fiber, and a diffraction grating such as a fiber grating. The semiconductor optical amplifier device has first and second end surfaces. The optical waveguide is optically coupled to the semiconductor optical amplifier device. The diffraction grating is optically coupled to the optical waveguide. The semiconductor optical amplifier device and the diffraction grating constitute an external cavity. An optical cavity length of the external cavity is in a range of 13 millimeters or more but 27 millimeters or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser module for optical communication.

2. Related Background of the Invention

A fiber grating laser is described in the following literature, which discloses the related art in this field: “Technical Report of IEICE, EMD2002-24, CPM2002-66, OPE2002-46, LQE2002-101 (2002-08), The Institute of Electronics, Information and Communication Engineers.” This fiber grating laser does not include a peltier element and is therefore a non-temperature controlled type device. The fiber grating laser exhibits temperature dependency of a wavelength and electric-current dependency thereof which are superior to those of a distributed feedback (DFB) semiconductor laser. An oscillation wavelength of the fiber grating laser is defined by a correlation between a longitudinal mode of an external optical cavity, which is constituted by a semiconductor optical amplifier device and a fiber grating, and a reflection characteristic-of the fiber grating. As shown inFIG. 2of the literature, the fiber grating laser is configured to include at least one longitudinal-mode line within a reflection bandwidth of the fiber grating. Moreover, as shown inFIG. 4of the literature, the fiber grating laser is small in size.

SUMMARY OF THE INVENTION

In the fiber grating laser, the longitudinal mode closest to the Bragg wavelength of the fiber grating oscillates. This Bragg wavelength has temperature dependency which is attributable to physical property values of a constituent material such as silica glass. A temperature characteristic of a fiber grating made of silica glass is about 10 picometers per degree, which does not substantially depend on an injection current to the semiconductor optical amplifier device. As a result, the fiber grating laser exhibits less dependency on variations in the injection current and in the ambient temperature as compared with a DFB semiconductor laser device.

However, in the fiber grating laser, the longitudinal mode of the oscillation wavelength relates to a length of an external cavity and varies in accordance with the temperature. In addition, the Bragg wavelength of the fiber grating also varies in accordance with the temperature. A rate of change in the longitudinal mode due to temperature is greater than a rate of change in the Bragg wavelength due to temperature. Resultantly, mode hopping occurs when the temperature changes. In the mode hopping, when the longitudinal mode corresponding to the oscillation wavelength changes from the current longitudinal mode to an adjacent longitudinal mode on a shorter wavelength side, the oscillation wavelength discretely changes from the current wavelength to an adjacent wavelength on the shorter wavelength side.

The occurrence of mode hopping may cause deterioration in a transmission characteristic. Therefore, in order to avoid the occurrence of mode hopping, an attempt has been made to approximate the temperature dependency of the Bragg wavelength of the fiber grating to the temperature dependency of the longitudinal mode of the fiber grating laser. Realization of so-called mode-hop-free tuning is studied. However, it is difficult to completely match the rate of change in the longitudinal mode of the fiber grating laser due to temperature with the rate of change in the Bragg wavelength of the fiber grating due to temperature, and in reality, the mode-hop-free tuning is achieved only in a limited temperature range. In addition, mode-hop-free tuning causes an increase in costs associated with complication in structure and material. Practical application of mode-hop-free tuning has been difficult from this viewpoint as well.

During mode hopping, two or more adjacent longitudinal modes of the fiber grating laser compete with one another. Observing an oscillation spectrum of the fiber grating laser that is exhibiting mode hopping, two or more adjacent longitudinal modes oscillate with equivalent intensities. In other words, the fiber grating laser oscillates in multimode.

In optical communication, single-mode light is used as signal light for long-distance communication. This is because unignorable waveform distortion occurs in a transmission pulse in long-distance communication due to the group delay of the multimode when using multimode light as the signal light, and thereby a transmission characteristic is deteriorated.

To obtain single-mode light at a practical ambient temperature from the fiber grating laser without being bothered by the mode hopping, a peltier element is used for adjusting the temperature of the semiconductor optical amplifier device. The use of the peltier element causes an increase in the costs of the fiber grating laser. It is hardly possible to use a temperature controlled type fiber grating laser unless an application thereof can accept such a cost increase.

Meanwhile, a low-cost light source has been desired for optical communication systems such as a metro system and an access system. One of candidates for the light source in these optical communication systems is an non-temperature controlled type (uncooled) fiber grating laser.

Accordingly, extensive studies have been conducted to realize the uncooled fiber grating laser. As a result, the inventors have reached an invention of a laser module which is usable as a light source for optical communication regardless of the occurrence of mode hopping.

It is an object of the present invention to provide a laser module for optical transmission, the laser module usable as a light source for long-distance communication regardless of the occurrence of mode hopping.

According to an aspect of the present invention, a laser module comprises an external cavity including (a) a semiconductor optical amplifier device having first and second end surfaces, (b) a grating fiber having an end and a diffraction grating, and (c) a lens for optically coupling the first end surface and the end together. Here, an optical cavity length of the external cavity is in a range of 13 millimeters or more but 27 millimeters or less.

According to another aspect of the present invention, a laser module comprises (a) a semiconductor optical amplifier device having first and second end surfaces, (b) a grating fiber having an end and a diffraction grating, and (c) a component-mounted member for configuring an external cavity by optically coupling the semiconductor optical amplifier device and the grating fiber together. Here, the component-mounted member includes an abutting surface on which the end of the grating fiber is abutted; the component-mounted member mounts the semiconductor optical amplifier device; and an optical cavity length of the external cavity is in a range of 13 millimeters or more but 27 millimeters or less.

According to still another aspect of the present invention, a laser module comprises an external cavity including a semiconductor optical amplifier device having first and second end surfaces, and a planar optical waveguide having an end and a diffraction grating. Here, an optical cavity length of the external cavity is in a range of 13 millimeters or more but 27 millimeters or less.

According to yet another aspect of the present invention, a laser module comprises a semiconductor optical amplifier device having first and second end surfaces, an optical waveguide having an end optically coupled to the semiconductor optical amplifier device, and a diffraction grating optically coupled to the optical waveguide. Here, the semiconductor optical amplifier device and the diffraction grating constitute an external cavity, and an optical cavity length of the external cavity is in a range of 13 millimeters or more but 27 millimeters or less.

Whereas an optical cavity length of an external cavity is about 6 mm in a conventional laser module, the above-described laser modules have longer lengths of the external cavities. Accordingly, longitudinal mode intervals become narrower, and group delay distortion caused by multimode oscillation during mode hopping is thereby reduced. Thus, a fine transmission state is maintained during the mode hopping, and therefore the above-described laser modules can be used as light sources regardless of the occurrence of mode hopping. In this way, according to the present invention, a laser module for optical communication is provided, which is usable as a light source regardless of the occurrence of mode hopping.

In the laser module of the present invention, the end of the grating fiber may be a lens-shaped end portion. The lens-shaped end portion can further strengthen the optical coupling between the semiconductor optical amplifier device and the grating fiber, and the characteristics of the laser module are thereby improved.

In the laser module of the present invention, the semiconductor optical amplifier device may include a converting portion for converting a spot size of light emitted from the semiconductor optical amplifier device. The converting portion can convert the spot size of the light emitted from the semiconductor optical amplifier device and thereby further strengthen the optical coupling between the semiconductor optical amplifier device and the grating fiber. Accordingly, the characteristics of the laser module are improved.

In the laser module of the present invention, the diffraction grating may have a reflection spectrum and a full width at half maximum of the reflection spectrum may be set to 0.4 nanometer or below. With this reflection spectrum, as compared with the reflectivities of two longitudinal modes in the vicinity of the Bragg wavelength in a mode hopping region, reflectivities at wavelengths of longitudinal modes other than the foregoing two longitudinal modes become smaller.

For this reason, with this full width at half maximum, laser oscillation of the longitudinal modes other than the two longitudinal modes (primary modes) in the vicinity of the Bragg wavelength is suppressed and becomes weaker by 10 decibels (dB) or above than the primary modes, thus becoming an ignorable level. In other words, by using the diffraction grating with the full width at half maximum of 0.4 nanometer (nm) or below, two-mode oscillation is established in the mode hopping region and multimode oscillation in excess of two modes is thereby suppressed. Therefore, it is possible to suppress the occurrence of signal light containing wavelength components in multimode in excess of two modes in the laser module. Moreover, it is possible to prevent an increase in transmission delay attributable to the wavelength components in multimode in excess of the two modes. In this way, deterioration in transmission is further reduced.

In the laser module of the present invention, the diffraction grating may have a reflection spectrum, and a longitudinal mode interval of adjacent longitudinal modes in the external cavity may be set to a value within a full width at half maximum of the reflection spectrum. In this laser module, a plurality of longitudinal modes exists in the full width at half maximum of the reflection spectrum. Therefore, the plurality of longitudinal modes oscillates simultaneously during mode hopping and the oscillation shifts smoothly to the adjacent mode. In this way, it is possible to suppress output variation during the mode hopping.

The laser module of the present invention can be realized in some aspects described below.

The laser module of the present invention may further comprise (d) a mounting component which mounts the semiconductor optical amplifier device, (e) a lens holding member which is supported by the mounting component and holds the lens, (f) a ferrule which holds the grating fiber, and (g) a ferrule holding member which holds the ferrule and is supported by the mounting component. Here, the grating fiber has a first portion provided with the diffraction grating, and a second portion of a pigtail shape.

Moreover, the laser module of the present invention may further comprise (d) a mounting component which mounts the semiconductor optical amplifier device, (e) a lens holding member which is supported by the mounting component and holds the lens, (f) a ferrule which holds a fiber stub provided with the diffraction grating, and (g) a ferrule holding member which holds the ferrule and is supported by the mounting component.

Furthermore, the laser module of the present invention may further comprise (d) a component-mounted member having a first region and a second region which are provided along a predetermined axis. Here, the semiconductor optical amplifier device is mounted in the first region of the component-mounted member, the grating fiber is mounted in the second region of the component-mounted member, and the second region of the component-mounted member includes first and second supporting surfaces which support side surfaces of the grating fiber.

Another aspect of the present invention is a method of generating an optical signal. A longitudinal mode of a laser cavity of a laser module changes in accordance with the temperature, and in the laser module, mode hopping occurs at one mode hopping temperature at least. The method includes (a) a step of applying an electric signal to the laser module, (b) a step of generating laser signal light containing first and second wavelength components corresponding to two adjacent longitudinal modes from a laser module when an ambient temperature of the laser module is a mode hopping temperature, and (c) a step of generating laser signal light corresponding to a single longitudinal mode from the laser module when the ambient temperature of the laser module is different from the mode hopping temperature. Here, an interval between the first wavelength component and the second wavelength component is 90 picometers or below.

Another aspect of the present invention is a method of generating an optical signal. A longitudinal mode of a laser cavity of a laser module changes in accordance with the temperature, and in the laser module, mode hopping occurs at one mode hopping temperature at least. The method includes (a) a step of generating laser signal light containing first and second wavelength components corresponding to two adjacent longitudinal modes from a laser module by applying an electric signal to the laser module when an ambient temperature of the laser module is a mode hopping temperature, and (b) a step of generating laser signal light corresponding to a single longitudinal mode from the laser module by applying an electric signal to the laser module when the ambient temperature of the laser module is different from the mode hopping temperature. Here, an interval between the first wavelength component and the second wavelength component is 90 picometers or below.

In these method, the interval between the first wavelength component and the second wavelength component is 90 picometers or below. Accordingly, distortion of the optical signal due to the group delay distortion of the light caused by the first and second wavelength components is small. Therefore, there is no deterioration in transmission attributable to the group delay distortion in the mode hopping region.

The above-described object, other objects, characteristics, and advantages of the present invention will become apparent more easily by the detailed description of the preferred embodiments of the present invention to be implemented below with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The knowledge of the present invention can be easily understood by considering the following detailed description with reference to the accompanying drawings presented herein as examples. Now, embodiments of a laser module of the present invention will be described with reference to the accompanying drawings. The same components are designated by the same reference characters when possible. Although the embodiments will be described in the case of an optical communication wavelength region of 1.55 micrometers, it is to be understood that the scope of the present invention is not limited to the foregoing, and that the present invention can be embodied in any arbitrary wavelength region.

First Embodiment

FIGS. 1A,1B,2A,2B, and24are views showing configurations of laser modules. Referring toFIGS. 1A,1B,2A, and2B, each of laser modules1a,1b,1c, and1dcomprises a semiconductor optical amplifier device3, an optical waveguide5such as an optical fiber, and a diffraction grating7such as a fiber grating. In addition, referring toFIG. 24, a laser module1icomprises a semiconductor optical amplifier device3, and a planar waveguide5having a diffraction grating7. The semiconductor optical amplifier device3has a first end surface3aand a second end surface3b. The optical waveguide5is optically coupled to the semiconductor optical amplifier device3. The diffraction grating7is optically coupled to the optical waveguide5. A portion between the second end surface3bof the semiconductor optical amplifier device3and the diffraction grating7constitutes an external cavity9. An optical cavity length L of the external cavity9is set in a range of 13 millimeters or more but 27 millimeters or less. In a conventional laser module, an optical cavity length of an external cavity is set to about 6 mm. However, the length of the external cavitys9is formed longer in each of the above-described laser modules. Accordingly, deterioration in transmission caused by waveform distortion due to the group delay in multimode oscillation during mode hopping is suppressed, and these laser modules are usable as light sources regardless of the occurrence of mode hopping.

In the following, with reference toFIGS. 1A,1B,2A, and2B, description will be made regarding the case of using a grating fiber having a diffraction grating as the optical waveguide5. Referring toFIG. 1A, the laser module1acomprises an external cavity9a. The external cavity9aincludes the semiconductor optical amplifier device3, the grating fiber5, and a lens11. The grating fiber5is an optical fiber having an end5aand the diffraction grating7which is disposed at a predetermined distance away from the end5a. The lens11optically couples the end5aof the grating fiber5to the first end surface3aof the semiconductor optical amplifier device3. An optical cavity length La of the external cavity9ais set in a range of 13 millimeters or more but 27 millimeters or less. The lens11can strengthen the optical coupling between the diffraction grating7and the semiconductor optical amplifier device3.

Referring toFIG. 1B, the laser module1bcomprises an external cavity9b. The external cavity9bincludes the semiconductor optical amplifier device3, the grating fiber5, and a lens11. The grating fiber5has an end5band the diffraction grating7which is disposed at a predetermined distance away from the end5b. The end5bincludes a lens-shaped end portion. The lens11optically couples the end5bof the grating fiber5to the first end surface3aof the semiconductor optical amplifier device3. An optical cavity length Lb of the external cavity9bis set in a range of 13 millimeters or more but 27 millimeters or less. The lens-shaped end portion can further strengthen the optical coupling between the semiconductor optical amplifier device3and the grating fiber5, and the characteristics of the fiber grating are thereby improved.

Each of the laser modules1aand1bcomprises a housing17. The housing17holds the semiconductor optical amplifier device3, the grating fiber5, and the lens11.

Referring toFIG. 2A, the laser module1ccomprises a component-mounted member13for constituting an external cavity9c. The external cavity9cincludes the semiconductor optical amplifier device3and the grating fiber5. The component-mounted member13includes a first region15aand a second region15b. The first region15aand the second region15bare disposed along a predetermined axis. The first region15amounts the semiconductor optical amplifier device3. The second region15bincludes an abutting surface15con which an end5cof the grating fiber S is abutted. An optical cavity length Lc of the external cavity9cis set in a range of 13 millimeters or more but 27 millimeters or less. The grating fiber5is positioned depending on the abutting surface15cand optically coupled to the semiconductor optical amplifier device3by butt joint.

This structure does not require an independent lens. Accordingly, cost reduction can be achieved as compared with a configuration which required a lens. Moreover, since the semiconductor optical amplifier device3and the grating fiber5are the only components that constitute the cavity. Accordingly, an optical axis alignment operation in assembly of the laser module becomes easier, and improvement in productivity and cost reduction are thereby achieved.

Referring toFIG. 2B, in the laser module1d, an end of the grating fiber5may be a lens-shaped end portion5d. An optical cavity length Ld of an external cavity9dis set in a range of 13 millimeters or more but 27 millimeters or less. The lens-shaped end portion5dcan further strengthen the optical coupling between the semiconductor optical amplifier device3and the grating fiber5, and the characteristics of the laser module are thereby improved.

With the component-mounted member13of the laser module1cor1d, the optical coupling of the grating fiber5to the semiconductor optical amplifier device3can be achieved by passive alignment. Each of the laser modules1cand1dhas a butt joint structure.

Since a low-cost member such as silicon bench can be used as the component-mounted member13in the structure of the laser module1cor1d, it is possible to reduce material costs. Moreover, it is possible to mount the semiconductor optical amplifier device3and the grating fiber5with high precision by the passive alignment, by means of forming alignment markers, grooves for disposing the components, and the like on the component-mounted member13in advance. Accordingly, it becomes unnecessary to carry out optical axis alignment between the semiconductor optical amplifier device3and the grating fiber5, which is accompanied by an optical axis alignment operation while causing the semiconductor optical amplifier device3to emit light. Thus, it is possible to save the time of mounting and thereby to reduce manufacturing costs. In this way, it is possible to achieve a reduction in the costs of the module.

Next, with reference toFIG. 24, description will be made regarding the case of using the planar waveguide having the diffraction grating7, as the optical waveguide5. Referring toFIG. 24, a laser module1iincludes an external cavity9i. The external cavity9iincludes the semiconductor optical amplifier device3and the planar optical waveguide5. The planar optical waveguide5is formed on a Si substrate, for example, and may essentially contain silica glass. The planar optical waveguide5has one end5iand the diffraction grating7which is disposed at a predetermined distance away from the end5i. An optical cavity length Lh of the external cavity9iis set in a range of 13 millimeters or more but 27 millimeters or less.

In each of these laser modules1a,1b,1c,1d, and1i, the semiconductor optical amplifier device3may further comprise a converting portion for converting a spot size of light emitted from the semiconductor optical amplifier device3. The converting portion can convert the spot size of light emitted from the semiconductor optical amplifier device3and thereby further strengthen the optical coupling between the semiconductor optical amplifier device3and the grating fiber5. Accordingly, the characteristics of the laser module are improved.

According to the laser module1a,1b,1c,1d, and1i, it is possible to achieve long-distance transmission at a transmission rate of about 1 gigabits per second (Gbps) without performing temperature adjustment of the laser module.

In each of these laser modules1a,1b,1c,1d, and1i, the semiconductor optical amplifier device3receives a transmission signal DINthrough lead terminals19aand19bof the laser module. The laser module generates signal light LOUTin response to the transmission signal DIN.

FIG. 3is a graph showing a change in an oscillation wavelength of a laser module due to temperature. InFIG. 3, a vertical axis indicates a wavelength of laser light and a horizontal axis indicates an ambient temperature of the laser module. Referring toFIG. 3, in a normal region R1, the laser module oscillates in an m-th longitudinal mode M1. In a normal region R3, the laser module oscillates in an m+1-th longitudinal mode M2. In a normal region R5, the laser module oscillates in an m+2-th longitudinal mode M3. Concerning each of these longitudinal modes, the oscillation wavelength becomes longer as the temperature increases. There is a first mode hopping region R2between the normal regions R1and R3. There is a second mode hopping region R4between the normal regions R3and R5.

FIGS. 4A and 4Bare views showing relations between reflection spectra presented by the diffraction grating of the laser module, and the longitudinal modes of the laser module in the normal region R1and in the mode hopping region R2, respectively. In each ofFIGS. 4A and 4B, a horizontal axis indicates a wavelength and a vertical axis indicates refrectivity of the diffraction grating. A reflection spectrum SR and a longitudinal mode line LM are shown in each ofFIGS. 4A and 4B.FIG. 5Ais a view showing an oscillation spectrum of a laser module operating in the normal region R1.FIG. 5Bis a view showing an eye pattern of optical transmission after optical transmission was performed using the laser module operating in the normal region R1. InFIG. 5A, a vertical axis indicates power, and a horizontal axis indicates the wavelength.FIG. 6Ais a view showing an oscillation spectrum of the laser module operating in the mode hopping region R2.FIG. 6Bis a view showing an eye pattern after optical transmission was performed using the laser module operating in the mode hopping region R2. InFIG. 6A, a vertical axis indicates the power, and a horizontal axis indicates the wavelength.

In the normal region R1, only the m-th longitudinal mode M1is located in the vicinity of the Bragg wavelength as shown inFIG. 4A. Accordingly, the m-th longitudinal mode M1oscillates in a single mode as shown inFIG. 5A. Therefore, no group delay distortion of an optical pulse caused by wavelength dispersion of an optical transmission line occurs. As a result, an eye pattern without jitter components is obtained as shown inFIG. 5B. In this optical transmission, there is no deterioration in transmission attributable to the group delay distortion.

Next, consideration is made regarding the case where the ambient temperature is increased to a temperature in the mode hopping region R2. A line of the longitudinal mode wavelengths of the laser module and the Bragg wavelength of the reflection spectrum of the diffraction grating are shifted to a longer wavelength side when the temperature is increased. However, a rate of change in the longitudinal mode due to temperature is greater than a rate of change in the Bragg wavelength due to temperature. Accordingly, a shift of the longitudinal mode is larger than a shift of the Bragg wavelength with respect to a change of certain magnitude in temperature. For this reason, the m-th longitudinal mode M1of which laser oscillation is being performed is relatively separated from the Bragg wavelength along with the temperature increase as shown inFIG. 4B. Accordingly, the reflectivity to be obtained from the diffraction grating is reduced and the laser oscillation becomes difficult. Meanwhile, the m+1-th longitudinal mode M2, which is adjacent to the m-th longitudinal mode M1on the shorter wavelength side, approaches the Bragg wavelength along with the temperature increase. Therefore, the reflectivity to be obtained from the diffraction grating is increased and the laser oscillation becomes easier. As a result, the longitudinal mode for oscillation is switched from the mode M1to the mode M2at a certain temperature increase. This is referred to as a mode hopping phenomenon.

In the temperature range where the mode hopping phenomenon is triggered, the mode which has oscillated dominantly in the single mode is weakened, and a plurality of longitudinal modes existing in the full width at half maximum of the reflection spectrum of the diffraction grating comes in mode competition with one another. Accordingly, the laser module oscillates in multimode.FIG. 6Ashows an oscillation spectrum of the laser module which oscillates in multimode in the mode hopping region R2. The primary longitudinal modes M1and M2are mainly oscillating. A symbol ΔRAMBDA denotes a wavelength difference between the longitudinal mode M1and the longitudinal mode M2. The longer an optical cavity length is, the smaller this wavelength difference becomes. The optical cavity length is the sum of values obtained by multiplying physical lengths of regions with different refractive indexs inside a cavity by the respective refractive indexs, and is an effective cavity length to be sensed by light.FIG. 6Bshows an eye pattern after optical transmission using signal light which oscillates in multimode in the mode hopping region R2. In this eye pattern, jitters caused by transmission delay between the modes M1and M2appear at a leading edge and a trailing edge thereof. In other words, group delay distortion occurs in a waveform of a transmission optical pulse in this optical transmission. However, if an interval between the adjacent longitudinal modes is small, waveform distortion caused by the transmission delay between the longitudinal modes is reduced. In the laser module of this embodiment, the cavity length is set in the range of 13 millimeters or more but 27 millimeters or less, and the wavelength difference ΔRAMBDA is 90 picometers or below in an optical communication wavelength region of 1.55 micrometers. Hence the waveform distortion of the signal light is sufficiently small even in the temperature region where the mode hopping occurs.

In the laser module, an upper limit frequency that can be modulated is reduced when the cavity length is increased. Therefore, prior to the present invention, an attempt to use a laser module having a relatively long cavity length has not been carried out. In this embodiment, it is possible to reduce the interval between the oscillation modes in the multimode oscillation during the mode hopping by increasing the cavity length, and the transmission delay between these oscillation modes is thereby reduced. Hence, the waveform distortion caused by the transmission delay is also reduced during the mode hopping.

FIG. 7is a graph showing frequency response characteristics of laser modules having various optical cavity lengths. A horizontal axis indicates the cavity length (unit: millimeters) and a vertical axis indicates the frequency response (unit: megahertz). The frequency response indicates an upper limit frequency in a 3-decibel bandwidth. In the graph, symbols “∘”, “▴”, and “*” indicate characteristics of the laser modules with direct bias currents of 20 milliamperes, 30 milliamperes, and 40 milliamperes, respectively. Groups G1, G2, G3, and G4indicate measurement values of the laser modules having the cavity lengths of 13 millimeters, 27 millimeters, 41.6 millimeters, and 51.8 millimeters, respectively.

Referring toFIG. 7, modulation is possible at a frequency of 1.25 gigabits per second (Gbps) when the cavity length is 27 millimeters and the bias current is 20 milliamperes or above. As a result of detailed experiments, this laser module turned out to be capable of achieving the optical transmission at 1.25 gigabits per second (Gbps) over an 80-kilometer (km) optical transmission line constituted by a silica-type optical fiber. In the case of the cavity length of 27 millimeters, the longitudinal mode interval is equal to or less than 50 picometers in an optical communication wavelength region of 1.55 micrometers. Accordingly, a transmission delay between the adjacent longitudinal modes is small. Therefore, distortion of transmission waveform caused by the group delay distortion is considerably reduced even in the mode hopping region, and fine transmission characteristics can be obtained. A power penalty in a bit error rate (BER) characteristic of 80-kilometer transmission is 1 decibel (1 dB) or below relative to a back-to-back transmission when BER is 10−10. This value is practically acceptable.

When the optical cavity length is set shorter than 27 millimeters, an upper limit frequency of transmission becomes higher. However, since the interval between the longitudinal modes becomes wider, the transmission delay between the adjacent longitudinal modes becomes larger. As a result of detailed experiments in the case of modulation at 1.25 Gbps, in the 1.55-micrometer region, the interval between the adjacent longitudinal modes was 90 picometers or below if the optical cavity length is 13 millimeters or above, and it was found that deterioration in the transmission waveform caused by the group delay distortion did not occur significantly when the mode interval was set in the above range or below. As a consequence, the optical transmission at 1.25 gigabits per second over a 40-kilometer optical transmission line constituted by a silica-type optical fiber is achieved by use of the laser module having the cavity length of 13 millimeters or above.

According to the above-described results of the experiments, if the optical cavity length is set in the range of 13 millimeters or more but the 27 millimeters or less, signal transmission over 40 kilometers at 1.25 gigabits per second is achieved including a mode hopping region. Therefore, the non-temperature controlled type (uncooled) laser module is applicable to an optical transmission system as mentioned above.

Moreover, according to the results of the experiment shown inFIG. 7, if the optical cavity length is set in a range of 13 millimeters or more to 51.8 millimeters or less, a laser module is provided, which is usable in an non-temperature controlled state at a modulation rate of 500 MHz or above including a mode hopping region. The measurement values of the longitudinal mode intervals in the groups G1, G2, G3, and G4in the 1.55-micrometer wavelength region are 90 picometers, 45 picometers, 29 picometers, and 23 picometers, respectively.

In a preferred embodiment, the interval between the adjacent longitudinal modes in the external cavity may be set to a value within the full width at half maximum of the reflection spectrum of the diffraction grating. According to this laser module, a plurality of longitudinal modes exists in the full width at half maximum of the reflection spectrum of the diffraction grating in the mode hopping region. Therefore, the plurality of modes oscillates simultaneously during the mode hopping and the oscillation shifts smoothly to the adjacent mode. In this way, it is possible to suppress output variation during the mode hopping.

Moreover, in another preferred embodiment, the full width at half maximum of the reflection spectrum of the diffraction grating may be set to 0.4 nanometer or below. According to this reflection spectrum, as compared with reflectivities of two longitudinal modes in the vicinity of the Bragg wavelength, reflectivities at wavelengths of longitudinal modes other than the foregoing two longitudinal modes becomes smaller. For this reason, with this full width at half maximum, the laser oscillations in the longitudinal modes other than the two longitudinal modes in the vicinity of the Bragg wavelength (primary modes) are suppressed and reduced by 10 decibels (dB) or above as compared with the primary modes, thus becoming an ignorable level. That is, the wavelength components in the other modes do not contribute to the optical transmission of the signal. In other words, by using the diffraction grating with the full width at half maximum of 0.4 nanometer or below, two-mode oscillation is established in the mode hopping region, and the multimode oscillation in excess of two modes is thereby suppressed. Therefore, it is possible to suppress the occurrence of signal light having the wavelength components in multimode in excess of two modes in the laser module. Moreover, it is possible to prevent an increase in group delay distortion caused by the wavelength components in multimode in excess of the two modes. In this way, deterioration in transmission is further reduced.

In each of the laser modules1a,1b,1c,1d, and1i, the semiconductor optical amplifier device3receives the transmission signal DINthrough the lead terminals19aand19bof the relevant laser module. The laser module generates the signal light LOUTin response to the transmission signal DIN. A method of generating an optical signal by use of any of the laser modules1a,1b,1c,1d, and1itakes the following procedures. This method includes a step of generating laser signal light containing first and second wavelength components corresponding to two adjacent longitudinal modes from a laser module by applying an electric signal to the laser module when an ambient temperature of the laser module is a mode hopping temperature, and a step of generating laser signal light corresponding to a single longitudinal mode from the laser module by applying an electric signal to the laser module when the ambient temperature of the laser module is different from the mode hopping temperature. Here, an interval between the first wavelength component and the second wavelength component is 90 picometers or below.

The longitudinal mode of the optical cavity of the laser module changes in accordance with the temperature, and in the laser module, mode hopping occurs at one mode hopping temperature at least. However, in this method, the interval between the first wavelength component and the second wavelength component is 90 picometers or below. Accordingly, distortion of the optical signal due to the group delay distortion caused by the first and second wavelength components is small. Therefore, fine transmission is maintained even in the mode hopping region.

Second Embodiment

FIG. 8is a perspective view showing a laser module according to a second embodiment. This module is a coaxial laser module of a pigtail type.FIG. 9is a cross-sectional view of the laser module taken along the I—I line inFIG. 8.

Referring toFIGS. 8 and 9, a laser module1eincludes a semiconductor optical amplifier device23, a grating fiber25, and a lens31. The grating fiber25has an end surface25aand a diffraction grating27which is disposed at a predetermined distance away from the end surface25a. The lens31optically couples the end surface25aof the grating fiber25to a first end surface23aof the semiconductor optical amplifier device23. The semiconductor optical amplifier device23, the grating fiber25, and the lens31constitute an external cavity29. The semiconductor optical amplifier device23, the grating fiber25, and the lens31are held by a housing33. In this way, the laser module le is configured such that an optical cavity length L of the external cavity29is set in a range of 13 millimeters or more but 27 millimeters or less. Since the length of the external cavity29is longer than that of a conventional one, deterioration in transmission due to waveform distortion caused by group delay in multimode oscillation during mode hopping is suppressed. Accordingly, the laser module1ecan be used as a light source regardless of the occurrence of mode hopping.

Moreover, the laser module1emay further include a mounting component37, a lens holding member39, a ferrule45, and a ferrule holding member35. The lens holding member39holds the lens31. The ferrule45is provided above the mounting component37and holds the grating fiber25. The ferrule holding member35holds the ferrule45. The grating fiber25includes a first portion25bincluding the diffraction grating27, and a second portion25cof a pigtail shape.

This laser module1eis suitable for configuring a laser module having a relatively long cavity length.

The housing33comprises the ferrule holding member35, the mounting component37such as a can package, and the lens holding member39such as a lens cap. The ferrule holding member35is mounted on the mounting component37.

The ferrule holding member35comprises a sleeve41and a ferrule holder43. The ferrule holder43holds the ferrule45, and this ferrule45holds the grating fiber25. The ferrule holder43holds the ferrule45such that the first portion25bof the grating fiber25extends in a direction of a predetermined axis Ax.

The mounting component37includes a base37aextending along a predetermined reference plane, and a mounting portion37bprovided on the base37a. A first surface37cis located on an opposite side to a second surface37d. The mounting portion37bincludes a mounting surface37fwhich extends along another reference plane intersecting the first surface37cof the base37a. The base37ahas several through holes37etherein extending from the first surface37cto the second surface37d, and lead terminals49ato49dpass through the respective through holes37e. A glass member47is provided between each of the lead terminals49bto49dand the base37a. The glass members47insulate the lead terminals49bto49dfrom the base37aand hermetically seal spaces between the lead terminals49bto49dand the base37a. The semiconductor optical amplifier device23is supported by the mounting surface37fof the mounting portion37bvia a mounting component50such as a submount.

The lens holding member39is placed on the mounting component37. The lens holding member39includes a sidewall portion39aextending in the direction of the predetermined axis Ax, and a ceiling potion39bfor holding the lens. The ceiling portion39bis provided at one end of the sidewall portion39a. The lens31is disposed at an opening39cprovided on the ceiling portion39b. The other end of the sidewall portion39ais positioned on the first surface37cof the mounting component37.

A monitoring semiconductor photodetector51is mounted on the mounting component37. The semiconductor photodetector51is optically coupled to an end surface23bof the semiconductor optical amplifier device23and monitors the light from the semiconductor optical amplifier device23.

The sleeve41is positioned on the mounting component37. The sleeve41includes a sidewall portion41aextending in the direction of the predetermined axis Ax and a supporting surface41bfor supporting the ferrule holder43. The supporting surface41bis provided at one end of the sidewall portion41a. The supporting surface41bextends along a plane intersecting the predetermined axis Ax. The other end of the sidewall portion41ais positioned on the first surface37cof the mounting component37. The length of the sidewall portion41aof the sleeve41is longer than the length of the sidewall portion39aof the lens holding member39.

The ferrule holder43is disposed on the sleeve41. The ferrule holder43includes a sidewall portion43aextending in the direction of the predetermined axis Ax and a flange portion43bfor positioning itself relative to the sleeve41. The sidewall portion43aincludes an inner side surface43cfor defining a region for inserting the ferrule45. The flange portion43bincludes a sliding surface43dfor positioning the ferrule holder43on the supporting surface41b.

By use of the above-described components, the low-cost and highly reliable laser module le of the pigtail type is provided.

In this laser module1e, as shown inFIG. 9, the optical cavity length L is given as the sum of an optical length L1of the semiconductor optical amplifier, an optical length L2between the semiconductor optical amplifier and the lens, an optical length L3of the lens, an optical length L4between the lens and one end of the grating fiber, and an optical length L5between the one end of the grating fiber and the diffraction grating.

In the laser module as described above, the can package includes a lead pin, for example. The semiconductor optical amplifier device is die bonded onto the submount with Au—Sn eutectic solder. The submount is die bonded onto the can package with Au—Sn eutectic solder. The monitoring semiconductor photodetector is die bonded onto the can package with Au—Sn eutectic solder. One end surface of the semiconductor optical amplifier device is provided with an AR coating film, and the other end surface thereof is provided with an HR coating film. The AR-coated one end surface of the semiconductor optical amplifier device is optically coupled to the lens. The semiconductor optical amplifier device and the monitoring semiconductor photodetector are connected to the lead pin via a bonding wire.

Moreover, in a preferred embodiment, as shown inFIGS. 8 and 9, the laser module1emay include an anti-reflection film48provided on one end surface45aof the ferrule45and the end surface25aof the grating fiber25. In this case, it is possible to effectively suppress reflected light from the end surface45aof the ferrule45and from the end surface25aof the grating fiber25to the semiconductor optical amplifier device23. Thus, an oscillation state of the fiber grating laser is more stabilized.

FIG. 10is a graph showing a temperature characteristic of an oscillation wavelength of the laser module. The Bragg wavelength, peak reflectivity, and a reflection bandwidth (the full width at half maximum) of this fiber grating are 1552.54 nanometers, 28 percent, and 0.4 nanometer, respectively. The dimensions L1to L5of the respective portions of the laser module are set to 1.11, 1.11, 2.43, 6.54, and 2.15 millimeters, respectively, and the optical cavity length L is 13.34 millimeters. The longitudinal mode interval ΔRAMBDA is given as the following formula.
(RAMBDAOC×RAMBDAOC)/2L

Here, RAMBDAOCdenotes the oscillation wavelength. The oscillation wavelength is almost equal to the Bragg wavelength of the fiber grating. Accordingly, it is calculated from the above formula that the longitudinal mode interval is about 90 picometers.

The temperature characteristic of the oscillation wavelength of this laser module is measured. Conditions of laser modulation are as follows: a transmission rate at 1.25 gigabits per second; and modulation at an extinction ratio of 10 dB by an NRZ signal of PRBS 223−1. The result is shown inFIG. 10.

Referring toFIG. 10, mode hopping occurs in the vicinity of a temperature of 28 degrees centigrade and in the vicinity of a temperature of 37 degrees centigrade. A rate of change in the oscillation wavelength due to temperature is 28 picometers per degree in the normal region. On the other hand, ambient temperature dependency of the oscillation wavelength of a semiconductor laser of which the temperature is controlled by a peltier element is far smaller than the foregoing.

Next, the transmission characteristic of this laser before and after the mode hopping in the vicinity of 28 degrees centigrade was evaluated. The laser was modulated at an extinction ratio of 10 decibels (dB) by use of an NRZ signal of PRBS 223−1 with a transmission rate of 1.25 gigabits per second. The result is shown inFIGS. 11A to 14C.

FIGS. 11A to 11Care views showing a bit error rate, an oscillation spectrum, and an eye pattern after transmission over 40 kilometers, respectively, in the normal region before reaching the mode hopping region. These characteristics were measured at a temperature of 27.8 degrees centigrade. Referring toFIG. 11A, a power penalty is at a practically acceptable level according to a difference between the transmission over 40 kilometers (the symbol □) and back-to-back transmission (symbol ∘). Referring toFIG. 11B, a single longitudinal mode oscillates at this temperature. As compared with the maximum peak of this oscillation spectrum, an adjacent peak value is smaller by about 20 dB. Referring toFIG. 11C, jitters are hardly observed.

The mode hopping region begins when the ambient temperature is increased above 27.8 degrees centigrade.FIGS. 12A to 12CandFIGS. 13A to 13Care views showing bit error rates, oscillation spectra, and eye patterns after transmission of 40 kilometers, each in the vicinity of the occurrence of the mode hopping. The characteristics inFIGS. 12A to 12Cwere measured at a temperature of 28.1 degrees centigrade. The characteristics inFIGS. 13A to 13Cwere measured at a temperature of 28.2 degrees centigrade. Referring toFIGS. 12B and 13B, at each of these temperatures, two longitudinal modes oscillate with a mode interval of about 90 picometers which is equal to the calculated value. Of these modes, the mode on the longer wavelength side is the mode which has been oscillated before the mode hopping, and the mode on the shorter wavelength side is the mode which serves as a main part of oscillation after the mode hopping. As compared with the maximum peak of the oscillation spectrum, an adjacent peak value is smaller by 3 to 4 decibels (dB) or thereabouts. When the ambient temperature changed from 28.1 degrees centigrade to 28.2 degrees centigrade, the intensity of longitudinal mode on the shorter wavelength side is increased, and the main part of the longitudinal mode for oscillation is switched to the longitudinal mode on the shorter wavelength side. In other words, the mode hopping occurs at this point. Referring toFIGS. 12C and 13C, although jitters are observed, clear waveform distortion is not generated in each case. Referring toFIGS. 12A and 13A, concerning the bit error rate, the power penalty is 1 decibel or below at a BER value of 10−10, which is at a practically acceptable level. That is, fine transmission characteristics are achieved even in the mode hopping region where the transmission characteristics are supposed to be the worst.

FIGS. 14A to 14Care views showing a bit error rate, an oscillation spectrum, and an eye pattern, respectively, in the case of entering the normal region again after passing through the mode hopping region by means of further increasing the ambient temperature. These characteristics were measured at a temperature of 28.3 degrees centigrade. Referring toFIG. 14A, the power penalty is at a practically acceptable level. Referring toFIG. 14B, a single longitudinal mode oscillates at this temperature. As compared with the maximum peak of this oscillation spectrum, an adjacent peak value is smaller by about 12 dB. Referring toFIG. 14C, jitters are hardly observed.

Based on the above-described results of the measurement, according to the laser module of this embodiment, deterioration in the transmission waveform due to the group delay distortion is not so much as to affect seriously the judgment of 1 and 0 of a signal pulse after transmission, even in the course of multimode oscillation during the mode hopping. This advantage is deemed attributable to the fact that the longitudinal mode interval is 90 picometers or less.

As it can be understood from the above description, the laser module of this embodiment is deemed capable of optical transmission over 40 kilometers or above at the transmission rate of 1.25 gigabits per second at an arbitrary temperature, regardless of whether in the mode hopping region or not. Accordingly, it is possible to realize a coaxial laser module of an non-temperature controlled and pigtail type which can achieve long-distance transmission.

Third Embodiment

FIG. 15is a perspective view showing a laser module according to a third embodiment. This module is a coaxial laser module of a receptacle type.FIG. 16is a cross-sectional view of the laser module taken along the II—II line inFIG. 15.

Referring toFIGS. 15 and 16, a laser module if includes a semiconductor optical amplifier device23, a fiber stub26, and a lens31. The fiber stub26has an end26aand a diffraction grating27which is disposed at a predetermined distance away from the end26a. The lens31optically couples the end26aof the fiber stub26to a first end surface23aof the semiconductor optical amplifier device23. The semiconductor optical amplifier device23, the fiber stub26, and the lens31constitute an external cavity30. The semiconductor optical amplifier device23, the fiber stub26, and the lens31are held by a housing61. In this way, the laser module if is configured such that an optical cavity length L of the external cavity30is set in a range of 13 millimeters or more but 27 millimeters or less. Since the length of the external cavity30of the laser module if is longer than that of a conventional one, deterioration in transmission due to group delay distortion attributable to multimode oscillation during mode hopping is suppressed. Accordingly, the laser module if can be used as a light source regardless of the occurrence of mode hopping.

Moreover, the laser module if may further include a mounting component37, a lens holding member39, a short ferrule71, and a ferrule holding member63. The short ferrule71is provided above the mounting component37and holds the fiber stub26. The ferrule holding member63holds the short ferrule71.

This laser module if is suitable for configuring a laser module having a relatively long cavity length.

The housing61includes the ferrule holding member63, the mounting component37such as a can package, and the lens holding member39such as a lens cap. The ferrule holding member63is mounted on the mounting component37.

The ferrule holding member63includes a sleeve65, a joint sleeve67and a ferrule holder69. The ferrule holder69holds the short ferrule71via a split sleeve73, and this short ferrule71holds the fiber stub26. The ferrule holding member63holds the short ferrule71so as to constitute a receptacle structure.

The sleeve65is positioned on the mounting component37. The sleeve65has a sidewall portion65aextending in a direction of a predetermined axis Ax, and an aligning surface65bor65cfor aligning the joint sleeve67(the aligning surface65bis used in this embodiment). The aligning surface65bor65cis an outer side surface or an inner side surface of the sidewall portion65a. This outer side surface or inner side surface extends in the direction of the predetermined axis Ax. The length of the sidewall portion65aof the sleeve65is longer than the length of a sidewall portion39aof the lens holding member39.

The joint sleeve67is disposed on the sleeve65. The joint sleeve67has a sidewall portion67aextending in the direction of the predetermined axis Ax, and a ceiling potion67bprovided at one end of the sidewall portion67a. The sidewall portion67ahas an aligning surface67cor67dfor aligning the joint sleeve67with the sleeve65(the aligning surface67dis used in this embodiment). The aligning surface67cor67dis an outer side surface or an inner side surface of the sidewall portion67a. This outer side surface or inner side surface extends in the direction of the predetermined axis Ax. The ceiling portion67bhas an opening67etherein. The light from the lens31reaches the end surface26aof the fiber stub26through the opening67e. The ceiling portion67bhas a sliding surface67f. The ferrule holder69is positioned on the sliding surface67f. The joint sleeve67can be slid along the sidewall portion65aof the sleeve65, and as a result of positioning by such a movement, a desired optical cavity length can be obtained. The ferrule holder69is disposed on the joint sleeve67. The ferrule holder69has a sidewall portion69aextending in the direction of the predetermined axis Ax and a flange portion69bfor positioning itself relative to the sleeve65. The sidewall portion69ahas an inner side surface69cfor defining a region for inserting the short ferrule71. The flange portion69bhas a sliding surface69dfor positioning the ferrule holder69on the sliding surface67f. Moreover, the ferrule holder69includes a concave portion69eon the inner side surface69cthereof. The concave portion69eis provided with the split sleeve73. The short ferrule71and the fiber stub26are positioned by use of the split sleeve73. The split sleeve73has a split73a.

In the case of the laser module if of a receptacle type, a tip of an optical connector such as an SC connector, an FC connector, or an MU connector, each provided with an optical fiber, is inserted into the split sleeve73. Then, the tip of the optical connector is closely attached to another end surface26bon the opposite side to the end surface26aof the fiber stub26and an end surface71bon the opposite side to another end surface71aof the short ferrule71, thereby extracting the light outside. Since the receptacle type does not have a light emission terminal of a pigtail shape, it is possible to downsize the laser module and thereby to facilitate incorporation thereof into a device or the like. Moreover, it is possible to take out an optical fiber to be connected to an emission terminal together with a connector. Accordingly, it is easy to change the type of an optical fiber if necessary and to replace an optical fiber which is defected due to breakage or the like.

Moreover, in a preferred embodiment, as shown inFIGS. 15 and 16, the laser module if may be provided with inclined surfaces, which are inclined relative to an optical axis of the fiber stub26, at the end26aof the fiber stub26and the end surface71aof the short ferrule71. Alternatively, as similar to the second embodiment, the laser module if may include an anti-reflection film on the end26aof the fiber stub26and on the end surface71aof the short ferrule71, instead of the inclined surfaces. It is to be noted that the laser module1eof the second embodiment may be provided with inclined surfaces, which are inclined relative to an optical axis of the grating fiber25, at the end surface25aof the grating fiber25and the end surface45aof the ferrule45. By the formation of the anti-reflection films or the inclined surfaces, it is possible to effectively suppress reflected light from the end surfaces25aand45a, or26aand71aof the grating fiber25and the ferrule45, or the fiber stub26and the short ferrule71, respectively, to the semiconductor optical amplifier device23. In this way, an oscillation state of the fiber grating laser is stabilized.

FIG. 17is a graph showing a temperature characteristic of an oscillation wavelength of the laser module. The Bragg wavelength, the peak reflectivity, and the reflection bandwidth (the full width at half maximum) of this fiber grating are 1530 nanometers, 30 percent, and 0.1 nanometer, respectively. The dimensions L1 to L5 of respective portions of the laser module inFIG. 16are set to 1.11, 1.11, 2.43, 6.54, and 15.77 millimeters, respectively, and the optical cavity length L is 26.96 millimeters. A calculated value of the longitudinal mode interval is about 43 picometers.

The temperature characteristic of the oscillation wavelength of this laser module is measured. Conditions of laser modulation are as follows: a transmission rate at 1.25 gigabits per second; and modulation at an extinction ratio of 10 decibels (dB) by an NRZ signal of PRBS 223−1. The result is shown inFIG. 17.

Referring toFIG. 17, mode hopping occurs in the vicinity of a temperature of 13 degrees centigrade and in the vicinity of a temperature of 34 degrees centigrade. A rate of change in the oscillation wavelength due to temperature is 20 picometers per degree in the normal region.

FIGS. 18A to 19Care views showing oscillation spectra at various temperatures in the vicinity of the mode hopping region, such as at 11.8 degrees, 12.2 degrees, 12.4 degrees, 12.6 degrees, 12.8 degrees, and 13.0 degrees, respectively. The longitudinal mode interval is around 45 picometers (pm), which substantially coincides with a calculated value. Referring toFIG. 18A, the laser module is in the normal region at 11.8 degrees centigrade and oscillates in a single longitudinal mode. The mode hopping region begins when the ambient temperature is increased. As apparent from the oscillation spectra at the temperatures of 12.2 degrees, 12.4 degrees, and 12.6 degrees, which are shown inFIG. 18B,FIG. 18C, andFIG. 19A, respectively, the intensities of two longitudinal modes become approximately equal. In a region at these temperatures, the laser module oscillates in multimode. Referring toFIG. 19B, at 12.8 degrees centigrade, the main part of the longitudinal mode for oscillation of the laser module is switched to an adjacent longitudinal mode on the shorter wavelength side. In other words, the mode hopping occurs at this point. Referring toFIG. 19C, at 13.0 degrees centigrade, the laser module is again in the normal region and oscillates in a single longitudinal mode.

FIGS. 20A to 20Cshow the results of transmission experiments in the mode hopping region.FIG. 20Ashows bit error rates in transmission of 40 kilometers (symbol □), in transmission of 80 kilometers (symbol), and in back-to-back transmission (symbol ◯).FIG. 20Bis a view showing an eye pattern after the transmission of 40 kilometers.FIG. 20Cis a view showing an eye pattern after the transmission of 80 kilometers. The measurement was carried out at a temperature of 12.2 degrees centigrade where the line width of the oscillation spectrum becomes the maximum and the multimode oscillation becomes most significant. Meanwhile, conditions of laser modulation are the same as those in the second embodiment. Referring toFIG. 20A, concerning the bit error rate, a power penalty in the transmission of 80 kilometers is 0.26 decibel or below at a BER value of 10−10, which is at a practically acceptable level. Referring toFIGS. 20B and 20C, although some jitters are observed, there is no waveform distortion which may affect the judgment of data1and0.

According to the above-described measurement results, deterioration in the transmission waveform due to group delay distortion is not so much as to affect the judgment of 1 and 0 of a signal pulse after transmission even when the laser module of this embodiment oscillates in multimode in the mode hopping region. This advantage is deemed attributable to the fact that the longitudinal mode interval is around 45 picometers (pm).

As it can be understood from the above description, the laser module of this embodiment is deemed capable of optical transmission over 80 kilometers or above at a transmission rate of 1.25 gigabits per second at an arbitrary temperature, regardless of whether in the mode hopping region or not. Accordingly, it is possible to realize a coaxial laser module of a non-temperature controlled and receptacle type which can achieve long-distance transmission.

Moreover, as it can be understood from the results of spectrum measurement shown inFIGS. 11A to 14C, andFIGS. 18A to 19C, in any of the second and third embodiments, the longitudinal modes other than the two modes in the center within the mode hopping region become weaker in intensity by 10 decibels (dB) or above as compared with the two longitudinal modes in the center. Accordingly, group delay distortion attributable to the other longitudinal modes is at an ignorable level. This is because no high reflection can be obtained from the fiber grating27for the modes other than the two modes in the closest vicinity of the Bragg wavelength since the full width at half maximum of the reflection spectrum of the fiber grating27is narrow, 0.4 nanometer (nm) or below, and so the oscillations in the other modes are considerably suppressed. Therefore, by using a fiber grating having the full width at half maximum of 0.4 nanometer (nm) or below, it is possible to establish two-mode oscillation in the mode hopping region and thereby to suppress the oscillation in multimode in excess of the two modes. Thus, it is possible to suppress the occurrence of signal light containing wavelength components in multimode in excess of two modes, in the laser module. Moreover, it is possible to suppress an increase in transmission delay caused by the oscillation components in multimode in excess of the two modes. In this way, deterioration in transmission is further reduced.

In the second and third embodiments, the end surface25aof the grating fiber25and the end surface26aof the fiber stub26may be lens-shaped end portions. Moreover, the semiconductor optical amplifier device23may include a converting portion for converting a spot size of the light emitted from the semiconductor optical amplifier device23. These structural modifications further strengthen the optical coupling between the semiconductor optical amplifier device23and the grating fiber25or the fiber stub26. Accordingly, the characteristics of the laser module are enhanced.

Fourth Embodiment

FIG. 21is a perspective view showing a laser module according to a fourth embodiment. This module is a laser module of a surface mounting type.FIG. 22is a cross-sectional view taken along the III—III line inFIG. 21.

Referring toFIGS. 21 and 22, a laser module1gincludes a semiconductor optical amplifier device23and a grating fiber24. The grating fiber24has an end surface24a, another end surface24b, and a diffraction grating27which is disposed at a predetermined distance away from the end surface24a. The end surface24aof the grating fiber24is optically coupled to a first end surface23aof the semiconductor optical amplifier device23. The semiconductor optical amplifier device23and the grating fiber24constitute an external cavity28. The semiconductor optical amplifier device23and the grating fiber24are held by a component-mounted member81such as a silicon bench. In this way, the laser module1gis configured such that an optical cavity length L of the external cavity28is set in a range of 13 millimeters or more but 27 millimeters or less. Since the length of the external cavity28of the laser module1gis longer than that of a conventional one, deterioration in transmission due to group delay distortion attributable to multimode oscillation during mode hopping is suppressed. Accordingly, the laser module1gcan be used as a light source regardless of the occurrence of mode hopping.

Moreover, the laser module1gmay further include the component-mounted member81and a ferrule83. The ferrule83holds the grating fiber24.

The component-mounted member81includes first, second and third regions81a,81b, and81c, which are serially arranged along a predetermined axis Ax. The semiconductor optical amplifier device23is mounted in the first region81a. The semiconductor optical amplifier device23can be accurately positioned in conformity to a positioning mark on the component-mounted member81. The second region81bincludes an optical fiber supporting portion85for supporting the grating fiber24. The optical fiber supporting portion85extends along the direction of the predetermined axis Ax, and has two side surfaces85aand85bfor supporting side surfaces of the grating fiber24. The third region81cincludes a ferrule supporting portion87for supporting the ferrule83. The ferrule supporting portion87extends along the direction of the predetermined axis Ax, and has two side surfaces87aand87bfor supporting side surfaces of the ferrule83. A positioning surface98ais provided between the first region81aand the second region81b. The positioning surface98ais provided by a positioning groove98which extends in a direction intersecting the predetermined axis Ax. The end surface24aof the grating fiber24is allowed to abut on the positioning surface98aand thereby positioned. An intermediate groove91extending in the direction intersecting the predetermined axis Ax is provided between the second region81band the third region81c.

The laser module1gfurther includes a holder member93such as a glass plate, and an adhesive member95. The holder member93is disposed on the second region81bof the component-mounted member81and fixes the grating fiber24placed on the optical fiber supporting portion85. The adhesive member95is used to fix the ferrule83placed on the ferrule supporting member87.

This laser module1gis suitable for configuring a laser module having a relatively long cavity length.

Moreover, a monitoring semiconductor photodetector97is mounted with high precision in the first region81aof the component-mounted member81with a marker. In addition, referring toFIG. 22, a resin body99(which is omitted inFIG. 21) is provided on the component-mounted member81in order to protect the semiconductor optical amplifier device23, the grating fiber24and the semiconductor photodetector97. The resin body99covers the semiconductor optical amplifier device23, the grating fiber24, and the semiconductor photodetector97. The resin body99provides optical coupling between the end surface23aof the semiconductor optical amplifier device23and the end surface24aof the grating fiber24, and optical coupling between the other end surface23bof the semiconductor optical amplifier device23and the semiconductor photodetector97. The resin body99can transmit the light from the semiconductor optical amplifier device23. The resin body99is silicon resin, for example, which is formed by potting. This resin body99enhances mechanical strength of the module and improves dampproof characteristics. Preferably, the resin body99has refractive index which is close to that of the grating fiber24. In this way, a quantity of reflected light from the end surface24aof the grating fiber24to the semiconductor optical amplifier device23is reduced, and the oscillation characteristic of the laser module1gis thereby stabilized.

The laser module1gincludes a lead frame101and, for example, a thermoplastic epoxy sealing resin103. The lead frame101mounts the assembled component-mounted member81. The lead frame101includes an island101aand several lead terminals101b. Using the sealing resin103, the component-mounted member81and the lead frame101are sealed by transfer molding.

Referring toFIG. 22, a symbol L1denotes an optical length of the semiconductor optical amplifier device23. A symbol L2denotes an optical interval between the end surface23aof the semiconductor optical amplifier device23and the end surface24aof the grating fiber24. Moreover, a symbol L3denotes an optical interval between the end surface24aof the grating fiber24and the diffraction grating27. The optical cavity length L of the external cavity28is provided as the sum of L1to L3. For example, it is possible to achieve a longer cavity length, for example, in a range of 13 millimeters or more but 27 millimeters or less by increasing the second and third regions81band81cof the component-mounted member81.

As it can be understood from the above description, the laser module of this embodiment is deemed capable of optical transmission of 40 kilometers or above at a transmission rate of 1.25 gigabits per second at an arbitrary temperature regardless of whether in the mode hopping region or not. Accordingly, it is understood that a laser module of a non-temperature controlled and surface mounting type capable of long-distance transmission can be realized.

Fifth Embodiment

FIG. 23is a cross-sectional view of a laser module taken along a line corresponding to the III—III line shown inFIG. 21. This module is another embodiment of the laser module of a surface mounting type. Referring toFIG. 23, a laser module1hincludes a semiconductor optical amplifier device23and a grating fiber24. The semiconductor optical amplifier device23and the grating fiber24are held by a component-mounted member81such as a silicon bench. In this way, the laser module1his configured such that an optical cavity length L of an external cavity32is set in a range of 13 millimeters or more but 27 millimeters or less to achieve a transmission rate of 1.25 Gbps. Since the length of the external cavity32of the laser module1his longer than that of a conventional one, deterioration in transmission due to group delay distortion caused by multimode oscillation during mode hopping is suppressed. Accordingly, the laser module1hcan be used as a light source regardless of the occurrence of mode hopping.

Moreover, the laser module1hmay further include the component-mounted member81such as a silicon bench, and a ferrule83, similarly to the laser module1gof the fourth embodiment.

The laser module1hmay include a housing105such as a mini dual-in-line (MINI-DIL) package. The component-mounted member81is disposed on a bottom portion105aof the housing105. After the housing105is filled with inactive gas such as nitrogen gas, the housing105with a metal lid putted thereon is subjected to seam welding. When a metallic film, e.g. an Au film, is provided on a side surface of the ferrule, the relevant portion is sealed by solder.

In the fourth and fifth embodiments, the end surface24aof the grating fiber24may be a lens-shaped end portion. Meanwhile, the semiconductor optical amplifier device23may include a converting portion for converting a spot size of the light emitted from the semiconductor optical amplifier device23. With these structural modifications, the optical coupling between the semiconductor optical amplifier device23and the grating fiber24is further strengthened. Accordingly, the characteristics of the laser module are improved.

In the fourth and fifth embodiments, it is possible to provide an anti-reflection film to the end surface24aof the grating fiber24. Alternatively, it is possible to provide an inclined surface, which is inclined relative to an optical axis of the fiber grating24, at the end surface24aof the grating fiber24. By the formation of the anti-reflection film or the inclined surface, it is possible to effectively suppress reflected light from the end surface24ato the semiconductor optical amplifier device23. In this way, an oscillation state of the laser module is more stabilized.

In the fourth and fifth embodiments, a full width at half maximum of a reflection spectrum of the grating fiber27may be set to 0.4 nanometer (nm) or below. With this reflection spectrum, as compared with reflectivities of two longitudinal modes (primary modes) in the vicinity of the Bragg wavelength, it is possible to render reflectivities at wavelengths of longitudinal modes other than these primary modes sufficiently small in a mode hopping region. For this reason, with this full width at half maximum, laser oscillations of the longitudinal modes other than the two longitudinal modes (primary modes) in the vicinity of the Bragg wavelength are suppressed and become weaker by 10 decibels (dB) or above than the primary modes, which is at an ignorable level. That is, wavelength components in the other modes do not contribute to optical transmission of a signal. In other words, by using the fiber grating with the full width at half maximum of 0.4 nanometer or below, two-mode oscillation is established in the mode hopping region and the oscillation in multimode in excess of two modes is thereby suppressed. Therefore, it is possible to suppress the occurrence of signal light having wavelength components in multimode in excess of two modes, in the laser module. Moreover, it is possible to prevent an increase in group delay distortion caused by the wavelength components in multimode in excess of the two modes. In this way, deterioration in transmission is further reduced.

In the fourth and fifth embodiments, by using positioning markers and grooves for component arrangement, which are formed on the component-mounted member81in advance, all of the semiconductor optical amplifier device23, the grating fiber24, the semiconductor photodetector97, and the like can be mounted with high precision by passive alignment. Therefore, it becomes unnecessary to carry out optical axis alignment between the semiconductor optical amplifier device23and the grating fiber24, and between the semiconductor optical amplifier device23and the semiconductor photodetector97, which is accompanied by an optical axis alignment operation while causing the semiconductor optical amplifier device23to emit light. Accordingly, it is possible to save the time of mounting and thereby to reduce manufacturing costs. In this way, it is possible to achieve a reduction in costs of the module.

As described above, by using the laser module of this embodiment, in other words, by using the laser module having the optical cavity length L of the external cavity in a range of 13 millimeters or more but 27 millimeters or less, it becomes possible to achieve optical transmission over 40 kilometers or above at a transmission rate of 1.25 gigabits per second, regardless of the occurrence of mode hopping. Therefore, long-distance transmission becomes possible by use of a non-temperature controlled type laser module. The laser module of this embodiment is applicable to long-distance transmission in an optical communication system such as a metro system or an access system. Moreover, the laser module of this embodiment can be used as a coarse wavelength division multiplexing (CWDM) light source in an optical communication system such as a metro system or an access system. If a laser module of a coaxial structure is used, a low-cost light source can be provided.

Although the principle of the present invention has been described with reference to the drawings in some preferred embodiments, it is obvious to those skilled in the art that various modifications in arrangements and in details are possible without departing from the principle of the present invention. It is to be understood that the present invention is not limited to the specific configurations which have been disclosed throughout the embodiments. Therefore, all modifications and changes are to be encompassed by the scope and spirit of the invention as defined in the appended claims.