Abstract:
An external cavity type tunable laser source has a laser diode which cooperates with a reflecting section to constitute an external cavity, and outputs laser light, a laser driving circuit which supplies a laser driving current to the laser diode, a random noise generator which generates a noise current whose current value varies at random, an amplitude control section which controls an amplitude of the noise current output from the random noise generator based on a light intensity of the laser light output from the laser diode, and a current superimposition section which superimposes a noise current whose amplitude is controlled by the amplitude control section on the driving current output from the laser driving circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2004-270897, filed on Sep. 17, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to an external cavity type tunable laser source having a laser diode which cooperates with a reflecting section to constitute an external cavity and outputs laser light, a laser driving circuit which supplies a laser driving current to the laser diode, and a random noise generator which superimposes a noise current whose current value varies at random on the driving current output from the laser driving circuit. The invention particularly relates to an external cavity type tunable laser source in which optical modulation can be performed irrespective of the optical power.  
         [0004]     2. Description of the Related Art  
         [0005]     An external cavity type tunable laser source can vary the wavelength over a wide range. In some of such external cavity type tunable laser sources, the spectral line width is 500 [kHz] or less at full width half maximum, or namely the spectral line width is narrow, and therefore the coherence is very excellent.  
         [0006]     In an actual measurement (for example, when an optical power (also, called a light intensity) is measured by an optical power meter), however, the excellent coherence allows interference noises to be generated, thereby causing a measurement error. Usually, there is a reflection point in a measurement system, and interference occurs between measurement light and reflected light. The optical path difference between measurement light and reflected light is often varied during measurement, and interference noises are generated to cause a light intensity of measurement light to be varied.  
         [0007]     As a method of performing optical modulation to widen the spectral line width and reduce interference noises, what is proposed are a configuration in which the length of an external cavity is varied (For example, see Japanese Patent No. 3,422,804.), and that in which random noises are superimposed on a laser driving current for driving a laser diode (For example, see JP-A-10-107354.).  
         [0008]      FIG. 4  is a diagram showing the configuration of an external cavity type tunable laser source as a related art (random noises are superimposed on a laser driving current) (For example, see JP-A-10-107354.). With reference to  FIG. 4 , an external cavity type tunable laser source in a Littman arrangement will be described as an example. An optical amplifier  10  has a laser diode  11 , a first lens  12 , and a second lens  13 . The laser diode  11  has an antireflection coating  11   a  at one end. The first lens  12  converts light emitted from the one end (the end face where the antireflection coating  11   a  is formed) of the laser diode  11  to collimating beam, and emits the collimating beam. The second lens  13  converts light emitted from the other end of the laser diode  11  to collimating beam and emits the collimating beam.  
         [0009]     A wavelength selecting section  20  has a diffraction grating  21 , a wavelength selecting mirror  22 , and mirror rotating section  23 , selects the wavelength of the laser beam emitted from the one end of the optical amplifier  10 , and feedbacks the selected light to the optical amplifier  10 . The diffraction grating  21  wavelength-disperses the light from the optical amplifier  10  and that from the wavelength selecting mirror  22 . The wavelength selecting mirror  22  reflects the light wavelength-dispersed by the diffraction grating  21 , to the diffraction grating  21 . The mirror rotating section  23  rotates the wavelength selecting mirror  22  to select the wavelength of the light which is to be fed back by the diffraction grating  21  to the optical amplifier  10 .  
         [0010]     A laser driving circuit  30  outputs a laser driving current for driving the laser diode  11 . A random noise generator  40  amplifies white noises such as thermal noises and shot noises, for example, noises generated by a Zener diode or the like, to a current of a predetermined amplitude, and outputs the amplified noise current. The current superimposition section  60  superimposes the noise current output from the random noise generator  40  on the driving current output from the laser driving circuit  30 . The laser diode  11  is driven by the laser driving current on which the noise current is superimposed.  
         [0011]     The operation of the thus configured laser source will be described. First, the operation of the optical system will be described.  
         [0012]     The light emitted from the one end of the laser diode  11  is converted to collimating beam by the first lens  12 , and then enters the diffraction grating  21 . The light entering the diffraction grating  21  is diffracted by the diffraction grating  21 , wavelength-dispersed to different angles depending on the wavelength, and then incident on the wavelength selecting mirror  22 . Among the light incident on the wavelength selecting mirror  22 , only the light of a desired wavelength is reflected to the diffraction grating  21  through the same optical path. The wavelength to be reflected through the same optical path is selected by the mirror rotating section  23 .  
         [0013]     The light incident on the diffraction grating  21  is again wavelength-dispersed. Only the light of the wavelength selected by the wavelength selecting section  20  is converged in the laser diode  11  by the first lens  12  to be fed back. The other end of the laser diode  11 , and the wavelength selecting mirror  22  form an external cavity, and perform laser oscillation.  
         [0014]     By contrast, the laser light emitted from the other end which is not provided with the antireflection coating  11   a  is converted to collimating beam by the second lens  13 , and emitted as the output light. The wavelength selecting mirror  22  is rotated by the mirror rotating section  23 , whereby the wavelength of the light fed back from the wavelength selecting section  20  to the optical amplifier  10  is made tunable, and a wavelength sweep of the output light is performed.  
         [0015]     Then, the operations of the driving circuit  30  and the random noise generator  40  will be described.  FIGS. 5A  to  5 C are views diagrammatically showing characteristics of the laser driving current, the noise current, and the spectral line width. As shown in  FIG. 5A , the driving circuit  30  outputs the laser driving current (the current value: Id) for driving the laser diode, and, as shown in  FIG. 5B , the random noise generator  40  supplies the noise current of a constant amplitude ΔI. The current superimposition section  60  superimposes the noise current on the laser driving current, and the resulting current is supplied to the laser diode  11 . In  FIGS. 5A and 5B , the abscissa indicates the time, and the ordinate indicates the current. In  FIG. 5C , the abscissa indicates the wavelength “λ”, and the ordinate indicates the light intensity “P”.  
         [0016]     As described in JP-A-10-107354, the variation of the laser driving current supplied to the laser diode  11 , i.e., the variation of the injection current involves variations of the light intensity of the output light and electron density of the laser diode  11 . The variation of the electron density involves the variation of the refractive index and the temperature variation, with the result that the frequency of the light is varied. Therefore, the laser diode  11  is driven by the laser driving current on which the noise current is superimposed, and as a result the output light of a wide spectral line width is obtained. As shown in  FIG. 5C , namely, a spectrum  101  is obtained in which the spectral line width is wider than a spectrum  100  in the case where the noise current is not superimposed.  
         [0017]     Also during a period when the wavelength sweep of the output light is performed by rotating the wavelength selecting mirror  22 , the noise current is superimposed on the laser driving current, whereby the output light in which optical modulation is performed over the whole wavelength range, and in which the spectral line width is wide is output.  
         [0018]     Japanese Patent No. 3,422,804 (paragraph [0014] to [0022], FIGS. 1 to 4) and JP-A-10-107354 (paragraph Nos. [0002] to [0005] and [0020] to [0031], FIGS. 1 and 2) are referred to as related art.  
         [0019]     Even when the laser driving current is constant, usually, the light intensity of the output light of the laser diode  11  is varied depending on the wavelength. FIG.  6  shows an example of the wavelength characteristic of the laser diode  11 . At a wavelength λmax, the light intensity of the output light is the maximum. In the vicinity of the wavelength λmax, the light intensity is flat, and, as advancing toward a shorter wavelength side and a longer wavelength side, the light intensity is rapidly attenuated. The specification of the spectral line width is defined at a predetermined wavelength, for example, the wavelength λmax, and a noise current satisfying the specification of the spectral line width is set. The specification of the spectral line width is set not over the whole wavelength range, but at the predetermined wavelength.  
         [0020]     Conventionally, the specification is determined while the vicinity of the wavelength λmax is set as a tunable range. Recently, however, the tunable range is further widened by request of the user.  
         [0021]     While the light intensity of the output light is varied depending on the wavelength, however, the amplitude ΔI of the noise current output from the random noise generator  40  is constant. Therefore, there arises a problem in that the laser oscillation is turned off at a wavelength where the laser oscillation threshold current is high. Specifically, as the light intensity is further reduced, the laser oscillation threshold current is higher, thereby producing a problem in that the laser oscillation is turned off and optical modulation cannot be performed.  
       SUMMARY OF THE INVENTION  
       [0022]     An object of the invention is to provide an external cavity type tunable laser source in which optical modulation can be performed irrespective of a light intensity of laser light.  
         [0023]     The invention provides a external cavity type tunable laser source, having: a laser diode which cooperates with a reflecting section to constitute an external cavity, and outputs laser light; a laser driving circuit which supplies a laser driving current to the laser diode; a random noise generator which generates a noise current whose current value varies at random; an amplitude control section which controls an amplitude of the noise current output from the random noise generator based on a light intensity of the laser light output from the laser diode; and a current superimposition section which superimposes a noise current whose amplitude is controlled by the amplitude control section on the driving current output from the laser driving circuit.  
         [0024]     In the external cavity type tunable laser source, the amplitude control section has: a storage section which stores a current value of the laser driving current and a light intensity of the laser light corresponding to the current value, for each wavelength of the laser light output from the laser diode; a calculating section which calculates an increasing/decreasing amount of an amplitude of the noise current based on a relationship between current values and light intensities stored in the storage section; and an amplitude adjusting section which attenuates or amplifies the amplitude of the noise current output from the random noise generator in accordance with a calculation result of the calculating section.  
         [0025]     In the external cavity type tunable laser source, the amplitude control section has: a storage section which stores an increasing/decreasing amount of an amplitude of the noise current, for each wavelength of the laser light output from the laser diode; and an amplitude adjusting section which attenuates or amplifies the amplitude of the noise current output from the random noise generator in accordance with the increasing/decreasing amount stored in the storage section.  
         [0026]     In the external cavity type tunable laser source, the amplitude control section has: a light receiving section which receives a part of the laser light output from the laser diode, to measure a light intensity; a calculating section which calculates an increasing/decreasing amount of an amplitude of the noise current based on the light intensity measured by the light receiving section; and an amplitude adjusting section which attenuates or amplifies the amplitude of the noise current output from the random noise generator in accordance with a calculation result of the calculating section.  
         [0027]     According to the external cavity type tunable laser source, the amplitude control section controls (increases or decreases) the amplitude of the noise current output from the random noise generator on the basis of the light intensity of laser light. Therefore, the laser oscillation is not turned off, and the degree of modulation of the laser light can be set to a desired value. As a result, optical modulation can be performed irrespective of the light intensity of the laser light.  
         [0028]     Furthermore, the light receiving section always measures the light intensity. Even when the current value of the laser driving current supplied from the laser driving circuit  30  is varied, therefore, the degree of modulation of the laser light can be set to the desired value. As a result, optical modulation can be performed irrespective of the light intensity of the laser light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a diagram showing the configuration of a first embodiment of the invention;  
         [0030]      FIG. 2  is a diagram showing the configuration of a second embodiment of the invention;  
         [0031]      FIG. 3  is a diagram showing the configuration of a third embodiment of the invention;  
         [0032]      FIG. 4  is a diagram showing the configuration of an external cavity type tunable laser source as a related art;  
         [0033]      FIGS. 5A  to  5 C are views showing characteristics of a laser driving current, a noise current, and a spectral line width of laser light of a laser diode  11 ; and  
         [0034]      FIG. 6  is a view showing relationships between the wavelength of laser light of the laser diode  11  and the light intensity. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Embodiments of the invention will be described with reference to the accompanying drawings.  
       First Embodiment  
       [0036]      FIG. 1  is a diagram showing the configuration of a first embodiment of the invention. The components which are identical with those of  FIG. 4  are denoted by the same reference numerals, and their description is omitted. Referring to  FIG. 1 , an amplitude control section  50  is newly disposed between the random noise generator  40  and the current superimposition section  60 .  
         [0037]     The amplitude control section  50  has a storage section  51 , a calculating section  52 , and an amplitude adjusting section  53 . The amplitude control section  50  receives the noise current from the random noise generator  40  to attenuate or amplify the amplitude ΔI of the noise current on the basis of the light intensity of the laser light output from the laser diode  11 . The resulting current is superimposed on the laser driving current in the current superimposition section  60 .  
         [0038]     The storage section  51  stores the current value of the laser driving current, and the light intensity of the laser light at the current value, for each wavelength of the laser light output from the laser diode  11 . The calculating section  52  calculates an increasing/decreasing amount of the amplitude ΔI of the noise current from relationships of current values and light intensities stored in the storage section  51 . The amplitude adjusting section  53  receives a calculation result from the calculating section  52 , and the noise current from the random noise generator  40 . In accordance with the calculation result, the amplitude adjusting section  53  attenuates or amplifies the amplitude ΔI of the noise current, and then outputs the noise current to the current superimposition section  60 .  
         [0039]     The operation of the thus configured laser source will be described.  
         [0040]     Characteristics of the current value and the light intensity for each wavelength are stored in the storage section  51 . Specifically, the laser driving circuit  30  outputs the laser driving current which has the constant current value Id irrespective of the wavelength of the output light. For each wavelength, for example, at the interval of 1 [nm], the light intensity of the output light of the laser diode  11  is measured. The relationship between the laser driving current and the light intensity for each wavelength is stored in the storage section  51 .  
         [0041]     Next, the current value Id is changed, and the relationship between the laser driving current and the light intensity for each wavelength is again stored in the storage section  51 . This operation is repeated. Preferably, the storage into the storage section  51  may be conducted at adjustment before shipment of the laser source or at calibration of the laser source.  
         [0042]     Then, the operation which is conducted in a usual state to perform the optical modulation to widen the spectral line width will be described.  
         [0043]     The laser driving circuit  30  outputs the laser driving current of a constant level irrespective of the wavelength of the output light. On the other hand, the amplitude control section  50  obtains the current value Id of the laser driving current output from the laser driving circuit  30 , and the wavelength selected by the wavelength selecting section  20 . The calculating section  52  reads out the light intensity corresponding to the current value Id and the wavelength, from the storage section  51 , and calculates the amplitude of a noise current at which the desired degree of modulation is attained, and obtains an increasing/decreasing amount.  
         [0044]     Specifically, a ratio (i.e., an S/N ratio) of the light intensity of the output light of the laser diode  11  and an amplitude at which the light intensity is varied by the noise current, or namely an amplitude of a noise current which satisfies the specification of the spectral line width. Since the amplitude of the noise current which enables optical modulation within a range where the laser oscillation of the laser diode  11  is not turned off is calculated, the specification of the spectral line width is not always requested to be satisfied at a wavelength where the light intensity is low.  
         [0045]     On the basis of the calculation result, the amplitude adjusting section  53  attenuates or amplifies the noise current of the random noise generator  40 , and the noise current which is adjusted to a desired amplitude is superimposed on the laser driving current. The laser diode  11  is driven by the laser driving current on which the noise current is superimposed. The optical system operates in the same manner as the laser source shown in  FIG. 4 , and therefore its description is omitted.  
         [0046]     As described above, the calculating section  52  calculates the amplitude of the noise current on the basis of the light intensity of the laser light which is read out from the storage section  51 . In accordance with the calculation result, the amplitude adjusting section  53  increases or decreases the amplitude of the noise current output from the random noise generator  40 . Therefore, the laser oscillation is not turned off, and the degree of modulation of the laser light can be set to a desired value. As a result, optical modulation can be surely performed irrespective of the light intensity of the laser light. Consequently, the spectral line width can be widened over the whole wavelength range.  
         [0047]     In some cases, the laser driving current is varied, and, for example, the current value is reduced. When the noise current remains to have a constant amplitude, there is a case where the laser driving current flows from the laser diode  11  to the laser driving circuit  30 , or in the direction (negative direction) opposite to the ordinary direction, and the laser diode  11  is reversely biased. As a result, there is a possibility that the laser diode  11  is broken. In the laser source shown in  FIG. 1 , however, the calculating section  52  calculates an increasing/decreasing amount of the amplitude of the noise current for each current value. Therefore, an excessive noise current is not superimposed on the laser driving current, and the laser diode is not broken.  
       Second Embodiment  
       [0048]      FIG. 2  is a diagram showing the configuration of a second embodiment of the invention. The components which are identical with those of  FIG. 1  are denoted by the same reference numerals, and their description is omitted. Referring to  FIG. 2 , a storage section  54  is disposed in place of the storage section  51  and the calculating section  52 . The storage section  54  stores an increasing/decreasing amount of the amplitude of the noise current, for each wavelength of the laser light output from the laser diode  11 .  
         [0049]     The operation of the thus configured laser source will be described.  
         [0050]     First, an increasing/decreasing amount of the amplitude of the noise current for each wavelength is stored in the storage section  54 . Specifically, the laser driving circuit  30  outputs the laser driving current which has the constant current value Id irrespective of the wavelength of the output light. For each wavelength, for example, at the interval of 1 [nm], the light-intensity of the output light of the laser diode  11  is measured. From the relationship between the laser driving current and the light intensity for each wavelength, an external apparatus such as a personal computer calculates an increasing/decreasing amount of the amplitude of the noise current, and the calculated increasing/decreasing amount is stored in the storage section  54 .  
         [0051]     Next, the current value Id is changed, and the relationship between the laser driving current and the light intensity for each wavelength is again stored in the storage section  54 . This operation is repeated. Preferably, the storage into the storage section  54  may be conducted at adjustment before shipment of the laser source or at calibration of the laser source.  
         [0052]     Then, the operation which is conducted in a usual state to widen the spectral line width. The operation is substantially identical with that of the laser source shown in  FIG. 1 , and the points of difference will be described. The amplitude adjusting section  53  reads out an increasing/decreasing amount corresponding to the current value Id and the wavelength, and attenuates or amplifies the noise current of the random noise generator  40 , and the noise current which is adjusted to have a desired amplitude is superimposed on the laser driving current. The laser diode  11  is driven by the laser driving current on which the noise current is superimposed.  
         [0053]     As described above, the amplitude of the noise current output from the random noise generator  40  is increased or decreased by the amplitude adjusting section  53  in accordance with the increasing/decreasing amount read out from the storage section  54 . Therefore, the laser oscillation is not turned off, and the degree of modulation of the laser light can be set to a desired value. As a result, optical modulation can be surely performed irrespective of the light intensity of the laser light. Consequently, the spectral line width can be widened over the whole wavelength range.  
         [0054]     In some cases, the laser driving current is varied, and, for example, the current value is reduced. When the noise current remains to have a constant amplitude, there is a case where the laser driving current flows from the laser diode  11  to the laser driving circuit  30 , or in the direction (negative direction) opposite to the ordinary direction, and the laser diode  11  is reversely biased. As a result, there is a possibility that the laser diode  11  is broken. In the laser source shown in  FIG. 2 , however, the storage section  54  stores an increasing/decreasing amount of the amplitude of the noise current for each wavelength. Therefore, an excessive noise current is not superimposed on the laser driving current, and the laser diode is not broken.  
       Third Embodiment  
       [0055]      FIG. 3  is a diagram showing the configuration of a third embodiment of the invention. The components which are identical with those of  FIG. 1  are denoted by the same reference numerals, and their description is omitted. Referring to  FIG. 3 , a light receiving section  55  and a calculating section  56  are disposed in place of the storage section  51  and the calculating section  52 .  
         [0056]     The light receiving section  55  receives part of the laser light output from the laser diode  11 , and measures the light intensity. The calculating section  56  calculates an increasing/decreasing amount of the amplitude of the noise current in accordance with the light intensity supplied from the light receiving section  55 .  
         [0057]     The operation of the thus configured laser source will be described.  
         [0058]     The laser driving circuit  30  outputs the laser driving current. The light output from the laser diode  11  is branched by a branch means such as an optical coupler or a half mirror, and one of branched light beams is supplied to the light receiving section  55 . In the light receiving section  55 , the input laser light is received by a photodiode, and a photocurrent which is proportional to the light intensity is output. The photocurrent is current-to-voltage converted by an IV converting circuit which is not shown, and further converted to a digital value by an AD converter which is not shown, to be output to the calculating section  56 .  
         [0059]     From the digital value which is supplied from the light receiving section  55  and proportional to the light intensity, the calculating section  56  calculates an increasing/decreasing amount of the amplitude of the noise current at this light intensity. On the basis of the calculation result, the amplitude adjusting section  53  attenuates or amplifies the noise current of the random noise generator  40 , and the noise current which is adjusted to a desired amplitude is superimposed on the laser driving current. The laser diode  11  is driven by the laser driving current on which the noise current is superimposed. The optical system operates in the same manner as the laser source shown in  FIG. 1 , and therefore its description is omitted.  
         [0060]     As described above, the light receiving section  55  measures the light intensity of the laser light, and the calculating section  56  calculates the amplitude of the noise current on the basis of the measurement result. In accordance with the calculation result, the amplitude adjusting section  53  increases or decreases the amplitude of the noise current output from the random noise generator  40 . Therefore, the laser oscillation is not turned off, and the degree of modulation of the laser light can be set to a desired value. As a result, optical modulation can be surely performed irrespective of the light intensity of the laser light. Consequently, the spectral line width can be widened over the whole wavelength range.  
         [0061]     In some cases, the laser driving current is varied, and, for example, the current value is reduced. When the noise current remains to have a constant amplitude, there is a case where the laser driving current flows from the laser diode  11  to the laser driving circuit  30 , or in the direction (negative direction) opposite to the ordinary direction, and the laser diode  11  is reversely biased. As a result, there is a possibility that the laser diode  11  is broken. In the laser source shown in  FIG. 3 , however, the light receiving section  55  always measures the light intensity. Therefore, an excessive noise current is not superimposed on the laser driving current, and the laser diode is not broken.  
         [0062]     The invention is not restricted to these embodiments, and may be configured in the following manner.  
         [0063]     In the laser source shown in  FIG. 1 , the configuration in which the current value of the laser driving current is varied to several kinds, and the light intensity at each of the current values is stored in the storage section  51  has been described. In the case where the laser driving circuit  30  outputs a current value within a predetermined range, only a light intensity with respect to one current value may be stored, and the amplitude control section  50  may not obtain the current value of the laser driving current.  
         [0064]     In the laser source shown in  FIG. 2 , the configuration in which the current value of the laser driving current is varied to several kinds, and the increasing/decreasing amount at each of the current values is stored in the storage section  54  has been described. In the case where the laser driving circuit  30  outputs a current value within a predetermined range, only an increasing/decreasing amount with respect to one current value may be stored, and the amplitude control section  50  may not obtain the current value of the laser driving current.  
         [0065]     In the laser sources shown in  FIGS. 1 and 2 , the configuration in which the value is stored at the interval of 1 [nm] has been described. Any wavelength interval may be employed, and the interval may not be uniform.  
         [0066]     In the laser sources shown in FIGS.  1  to  3 , the configuration in which an external cavity type tunable laser source in a Littman arrangement is used has been described. The external cavity may have any configuration. For example, only a mirror serving as a reflecting means is disposed in the wavelength selecting section  20 , and the mirror may be moved along the optical axis. Alternatively, only a diffraction grating serving as a reflecting means may be disposed in the wavelength selecting section, and the diffraction grating may be moved along the optical axis.