Wavelength stabilizing apparatus and method of adjusting the same

A quartz etalon having light reflecting film layers on the Z-cut surfaces and electrode layers on the X-cut surfaces are axially supported in the X-cut surface electrode layers. A dither signal is applied to this axially supported portion to resonate the etalon. In this state, laser-beam is transmitted through the Z-cut surfaces of the etalon. The transmitted light is photo-electrically converted and subjected to synchronous detection by a dither signal. On the basis of an error signal obtained by the detection output, the oscillation wavelength of the semiconductor laser is controlled. Since the etalon resonates as its axially supported central portion functions as a node, the mechanical loss is small, and the Q value upon mechanical resonance is extremely large. This makes the synchronous detection output about 100 times as large as the conventional value. This increases the signal-to-noise ratio of the photoelectric conversion signal, and increases the wavelength accuracy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-304620, filed Oct. 18, 2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength stabilizing apparatus for stabilizing the wavelength of laser-beam of a semiconductor laser, and a method of adjusting the wavelength to be stabilized.

2. Description of the Related Art

A semiconductor laser is generally used as a light source for wavelength division multiplexing transmission. However, a semiconductor laser has relatively low temperature stability, so the wavelength of radiated light easily shifts due to environmental changes. This wavelength shift interferes with an adjacent wavelength. Therefore, a wavelength division multiplexing transmission system conventionally uses a wavelength stabilizing apparatus for stabilizing the wavelength of the laser-beam of a semiconductor laser.

As an example of this wavelength stabilizing apparatus, Jpn. Pat. Appln. KOKAI Publication No. 3-72686 discloses an apparatus using a quartz etalon resonator. In a wavelength stabilizing apparatus of this type, layers of light reflecting films are formed on the Z-cut surfaces of a quartz etalon, and electrodes are formed on the X-cut surfaces. A dither signal is applied to the electrodes to cause the quartz etalon to vibrate. In this state, laser-beam is transmitted through the quartz etalon. The laser-beam modulated by the cavity length variation of the etalon is received by a photo-detector and converted into an electrical signal. The detected electrical signal is then subjected to synchronous detection by using a dither signal, thereby generating an error signal. On the basis of this error signal, the injection current or temperature of the semiconductor laser is controlled. In this manner, the wavelength of light generated by the semiconductor laser is stabilized at the extremal value of the light transmittance of the quartz etalon.

In the conventional wavelength stabilizing apparatus, the quartz etalon is fixed to a base by an adhesive or the like. Since this increases the mechanical loss, the Q value of the mechanical resonance decreases. This makes it impossible to increase the optical modulation index of the laser-beam transmitted through the etalon. Accordingly, the signal-to-noise ratio of the electrical signal obtained from the photo-detector decreases, and as a consequence the wavelength detection accuracy decreases.

In addition, the frequency of the dither signal to be applied to the quartz etalon must be the same as the mechanical resonance frequency of the etalon itself. This requires precise frequency adjustment. Furthermore, in order that the wavelength characteristic, i.e., the light transmittance of the quartz etalon have an extremal value, the angle (elevation angle) to the optical axis must be adjusted (generally, the elevation angle is 0° at the extremal value).

Unfortunately, even if the angle of the quartz etalon is accurately adjusted, the end face of the quartz etalon is fixed to the base with adhesive after angle adjustment. Therefore, the adjusted angle changes by cure shrinkage of the adhesive or the like, and this produces an error between the wavelength to be stabilized and the extremal value. Also, no dither signal can be applied to the quartz etalon by connecting a line to it unless the adhesive cures. This makes accurate determination of the wavelength difficult. Accordingly, the workability of wavelength setting worsens, and the wavelength setting accuracy significantly decreases.

As described above, the conventional wavelength stabilizing apparatus has the problems that the mechanical loss of the quartz etalon is large, the optical modulation index is difficult to increase, the signal-to-noise ratio of the signal obtained from the photo-detector is low, and the wavelength detection accuracy is low. Also, the frequency of the dither signal to be applied to the quartz etalon must be the same as the mechanical resonance frequency, and this requires frequency adjustment. In addition, although the quartz etalon is fixed by adhesive after angle adjustment is performed to adjust the wavelength characteristic of the quartz etalon, the adjusted angle changes by cure shrinkage of the adhesive or the like. Furthermore, the workability of wavelength setting is low, and the wavelength setting accuracy is also very low.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wavelength stabilizing apparatus and wavelength stabilizing method capable of significantly increasing the wavelength detection accuracy, eliminating adjustment of a dither signal frequency, improving quartz etalon angle adjustment for adjusting the wavelength stabilizing characteristic, and also improving the workability and wavelength setting accuracy.

According to first aspect of the present invention, there is provided a wavelength stabilizing apparatus for stabilizing the wavelength of a laser-beam radiated from a semiconductor laser, comprising: an etalon which is obtained by cutting a quartz bulk, and includes a pair of light reflecting film layers formed on a pair of Z-cut surfaces of the etalon, respectively, and a pair of electrode layers formed on a pair of X-cut surfaces thereof, respectively; a dither signal generator which generates a dither signal corresponding to a mechanical resonance frequency of the etalon; a pair of shafts which are conductive, have a pair of end portions connected to the pair of X-cut surfaces of the etalon, respectively, to axially support the etalon, and are supplied with the dither signal; a supporting device which supports the pair of shafts; a photo-detector which receives the laser-beam transmitted through the etalon and converts the received laser-beam into an electrical signal; an error signal generator which generates an error signal by comparing the electrical signal with the dither signal; and a controller which controls a driving state of the semiconductor laser on the basis of the error signal, to approach the wavelength of the laser-beam to an extremal value of a light transmittance of the etalon.

According to second aspect of the present invention, there is provided a wavelength stabilizing method of stabilizing a wavelength of a laser-beam radiated from a semiconductor laser, comprising: axially supporting an etalon which is obtained by cutting a quartz bulk, and includes a pair of light reflecting film layers formed on a pair of Z-cut surfaces of the etalon, respectively, and a pair of electrode layers formed on a pair of X-cut surfaces thereof, respectively, in a pair of X-cut surfaces; oscillating the etalon by applying a dither signal to the axially supported portion of the etalon; sending the laser-beam into Z-cut surfaces of the etalon and passing the laser-beam between the reflecting film layers; receiving the laser-beam transmitted through the etalon, and converting the received laser-beam into an electrical signal; generating an error signal by comparing the electrical signal with the dither signal; and controlling a driving state of the semiconductor laser on the basis of the error signal, to approach the wavelength of the laser-beam to an extremal value of a light transmittance of the etalon.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

FIG. 1is a perspective view schematically showing a wavelength stabilizing apparatus as the first embodiment according to the present invention. InFIG. 1, reference numeral11denotes a semiconductor laser (LD). The semiconductor laser11oscillates by an injection current from a laser driver12, and emits signal light having a predetermined wavelength forward and backward. The signal light emitted from the front of the semiconductor laser11is sent into an optical fiber transmission path13by an optical system (not shown). The signal emitted from the back of the semiconductor laser11enters a wavelength stabilizing apparatus14of this embodiment.

In the wavelength stabilizing apparatus14, the signal light (to be referred to as laser-beam hereinafter) from the semiconductor laser11is converted into collimated light by a collimator141inserted on the optical axis. This collimated light is transmitted through a central portion of a quartz etalon142, and sent into a photo-detector (PD)143where the collimated light is converted into an electrical signal.

The quartz etalon142is obtained by cutting a quartz bulk into the shape of a rectangular parallelepiped, forming light reflecting film layers A1and A2on a pair of Z-cut surfaces (perpendicular to the optical axis), and forming electrode layers B1and B2on a pair of X-cut surfaces (parallel to the optical axis). The electrode layers B1and B2are connected to an oscillator144. The oscillator144generates a dither signal corresponding to the mechanical resonance frequency of the quartz etalon142. Therefore, the quartz etalon142resonates at the mechanical resonance frequency by the dither signal applied between the electrode layers B1and B2. The incoming laser-beam to the quartz etalon142is transmitted after being modulated by the cavity length variation of the quartz etalon142.

The photo-detector143receives the laser-beam modulated by the quartz etalon142, and generates an electrical signal corresponding to the change in light intensity. This electrical signal (to be referred to as a photo-detection signal hereinafter) is supplied to a synchronous detector145together with the output dither signal from the oscillator144. The synchronous detector145performs synchronous detection for the photo-detection signal and dither signal, and generates a phase error signal. This phase error signal is supplied to a wavelength controller146of the semiconductor laser11.

On the basis of the phase error signal, the wavelength controller146controls an injection current which drives the semiconductor laser11, or controls the temperature by using a Peltier element or the like. Consequently, the wavelength of the light of the semiconductor laser11stabilizes at the extremal value of the light transmittance of the quartz etalon142. If the semiconductor laser11is a wavelength-variable laser having a wavelength control electrode, a control signal based on the phase error signal is supplied to the wavelength control electrode.

The arrangement described above is the basic arrangement. The characteristic feature of the arrangement of the present invention is that the quartz etalon142is not directly fixed to a base147, but is axially supported in central portions of the X-cut surfaces. More specifically, one end portion of each of shafts148and149made of conductive wires is connected to the center of a corresponding one of the X-cut surface electrode layers of the quartz etalon142, such that the shafts148and149perpendicularly protrude from the X-cut surfaces. The shafts148and149are fixed to a pair of pillars14aand14b, respectively, formed on the base147, so that the quartz etalon142is separated from the base147and the optical axis of the laser-beam is positioned in the center of the quartz etalon142. The other end portions of the shafts148and149are connected to the oscillator144. Therefore, the dither signal generated by the oscillator144is supplied to the electrode layers of the quartz etalon142via the shafts148and149.

The operation of the wavelength stabilizing apparatus having the above arrangement will be described below with reference toFIG. 2.FIG. 2is a graph showing the light transmittance and synchronous detection output characteristic of the apparatus shown inFIG. 1. Referring toFIG. 2, a waveform A shows the way the light transmittance of the quartz etalon142changes by the dither signal.

When the dither signal is applied to the electrode layers on the X-cut surfaces, the quartz etalon142expands and contracts by the piezoelectric effect. Accordingly, the light transmittance also moves from side to side, albeit subtly, by the dither signal. As a consequence, the intensity of the laser-beam transmitted through the quartz etalon142changes. The sign of the slope of the light transmittance in a wavelength region W1from a minimum value Ab to a maximum value Ap of the light transmittance differs from that in a wavelength region W2from the maximum value Ap to the minimum value Ab. Therefore, the intensity of the transmitted light modulated by the quartz etalon142has a phase difference of 180° between the wavelength regions W1and W2.

Accordingly, synchronous detection is performed between a signal obtained by photo-electrically converting the transmitted light from the quartz etalon142and the dither signal, thereby obtaining a signal having a waveform B as an error signal. The error signal B is input to the wavelength controller146to control the injection current or temperature of the semiconductor laser11, thereby making the oscillation wavelength of the semiconductor laser11approach zero-crossing points B1and B2of the error signal B. The wavelengths at B1and B2are the maximum value Ap and minimum value Ab, respectively, of the light transmittance of the quartz etalon142. Therefore, by changing the polarity of feedback control, the wavelength of the laser-beam can be selectively stabilized at the wavelength which gives a maximum value or minimum value of the light transmittance of the quartz etalon142. This method using the dither signal as described above can stabilize the wavelength with high accuracy, since the method is unaffected by a DC drift of an amplifier or a dark current change of a photo-detector.

The foregoing is the basic operation principle of the wavelength stabilizing apparatus using a dither signal, and is similar to that of the conventional apparatus. However, the arrangement of the above embodiment has an effect of greatly increasing the synchronous detection output compared to that of the conventional apparatus. The reason will be explained below.

The quartz etalon142is separated from the base147, and the central portion of the quartz etalon142is axially supported by the shafts148and149. In a mechanical resonance mode at the lowest mechanical oscillation frequency in this state, the central portion of the quartz etalon142is a node, and the two end portions of the quartz etalon142are antinodes. Since the shafts148and149are connected to the node, therefore, the mechanical loss caused by the connection of the shafts148and149is small, and the Q value significantly increases upon mechanical resonance. In an experiment, when the quartz etalon142is fixed to the base147, the Q value of the quartz etalon142takes a number of several to several tens. In contrast, when the central portion of the quartz etalon142is axially supported, the Q value is several hundred to several thousand.

The modulation index of the quartz etalon transmitted light is a maximum in the node portion, i.e., in the central portion, and is 0 in the free end portions. Conventionally, the end portion of the quartz etalon is fixed to the base, so the node of the etalon is this end portion fixed to the base. Since it is difficult to pass laser-beam through this end portion, laser-beam is conventionally passed through a portion slightly above the end portion. A factor like this also decreases the modulation index of the transmitted light from the quartz etalon.

In contrast, in the arrangement of this embodiment, laser-beam can be transmitted through the central portion which is the node in which the optical modulation index is a maximum. Accordingly, the output from the synchronous detector145can be increased to be about 100 times as large as that in the conventional apparatus. This sharply increases the signal-to-noise ratio of the signal obtained from the photo-detector143, and thereby greatly increases the wavelength accuracy.

Second Embodiment

When a quartz etalon is fixed to a base as in the conventional apparatus, a small Q value makes the formation of an oscillation circuit difficult. In the first embodiment, however, the Q value of the mechanical resonance of the quartz etalon142significantly increases. Therefore, the quartz etalon142can be incorporated as an transducer into an oscillation circuit.

FIG. 3is a perspective view schematically showing a wavelength stabilizing apparatus as the second embodiment according to the present invention. The same reference numerals as inFIG. 1denote the same parts inFIG. 3, and a repetitive explanation thereof will be omitted.

In this apparatus shown inFIG. 3, a transducer V1which is a quartz etalon142is interposed between the base and collector of a transistor Q1, and capacitors C1and C2are connected in its base-to-emitter path and collector-to-emitter path, respectively, thereby forming a Colpitts oscillation circuit as a whole. An oscillation signal is naturally applied to the quartz etalon142as well.

In the oscillation circuit having the above arrangement, the oscillation frequency is higher by about 0.1% than the mechanical resonance frequency of the quartz etalon142, i.e., this oscillation frequency can be regarded as substantially the same as the mechanical resonance frequency. In this embodiment, therefore, the frequency of a dither signal can be automatically matched with the mechanical resonance frequency of the quartz etalon142without any adjustment. Synchronous detection can be performed by extracting an oscillation output signal from the collector of the transistor Q1, and applying the extracted signal to a synchronous detector145.

The above embodiment is explained by taking a Colpitts oscillation circuit as an example. However, the present invention can be similarly practiced by another type of oscillation circuit such as a Hartley oscillation circuit.

Third Embodiment

As the third embodiment of the present invention, a method of increasing the resistance against external vibration and shock will be described.

FIG. 4shows a structure when a portion in which a quartz etalon142is axially supported on a base147is viewed from the front with respect to the optical axis in a wavelength stabilizing apparatus14shown inFIG. 1or3. Referring toFIG. 4, the distance between pillars14aand14bis L cm, and the quartz etalon142is positioned in the center between the pillars14aand14b. Letting F be the shafting resonance frequency when shafts148and149are regarded as elastic members and the quartz etalon142is regarded as a material particle having a weight of W kg,

where E is a Young's modulus (kgf/cm2), g is a gravitational acceleration of 980 cm/s2, and I is the sectional secondary moment. If the shafts148and149are circular pillars and their diameter is d cm,
I=πd4/64  (2)

As shown inFIG. 5, the frequency component of a floor impulsive sound (vibration energy) is inversely proportional to a frequency of 300 Hz or higher. That is, the energy of a high frequency component significantly decreases; the energy decreases to small fractions at 10 kHz or higher, when compared to a low-frequency region. Accordingly, if the shafting resonance frequency F is set at 10 kHz or higher, the influence of external vibrational shock can be well reduced. To this end, it is effective to decrease the pillar-to-pillar distance L when equation (1) is taken into consideration.

FIG. 6is a graph showing a calculation example of the shafting resonance frequency F as a function of the distance L. Since the quartz etalon142has a width, the distance L cannot be made smaller than this width of the quartz etalon142. However, when the distance L is set at 0.4 cm, the shafting resonance frequency F is 10 kHz or higher, i.e., a high enough shafting resonance frequency can be set.

Fourth Embodiment

As the fourth embodiment according to the present invention, a method of maximizing the wavelength detection sensitivity of the bottom of the light transmittance will be described in detail below.

As is well known, a light transmittance T of a Fabry-Perot resonator using a quartz etalon142is represented by

where R is the reflectance of light reflecting films B1and B2, n is the refractive index of the quartz etalon142, and L is the length of the quartz etalon142in the optical axis direction.

The characteristic of the light transmittance T is as indicated by a waveform A shown inFIG. 2. An output from a synchronous detector145is as indicated by a waveform B shown inFIG. 2. This is the form of first derivative dT/dL of equation (3). The slope of dT/dL represents the wavelength detection sensitivity. Normally, detection sensitivity |dT/dL| when the light transmittance takes an extremal value is smaller than that when the light transmittance takes a minimum value. Therefore, it is desirable to maximize the detection sensitivity |dT/dL| when the light transmittance takes a minimum value. Conditions for the purpose are obtained from d2T/dL2=0. Accordingly, equation (3) is rewritten to obtain a quadratic equation represented by
R2−6R+1=0  (4)

When this quadratic equation is solved, the reflectance R is obtained as R=3−2√{square root over ( )}2=0.172. That is, when the reflectance is 17%, the wavelength detection sensitivity of the bottom of the light reflectance is a maximum. In practice, if the reflectance falls within the range of about 17±5%, the detection sensitivity does not significantly deteriorate.

Fifth Embodiment

FIG. 7is a perspective view schematically showing a wavelength stabilizing apparatus as the fifth embodiment according to the present invention. The same reference numerals as inFIG. 1denote the same parts inFIG. 7, and a repetitive explanation thereof will be omitted.

In this embodiment, an adjusting method will be explained by which a wavelength to be stabilized is matched with the extremal value of the light transmittance of a quartz etalon142by adjusting the elevation angle of the quartz etalon142with respect to the optical axis of laser-beam.

When the quartz etalon142is inclined a very small angle α from a plane perpendicular to the optical axis, a light transmittance T is represented by

Therefore, the wavelength characteristic can be freely shifted by changing the angle α. In actual adjustment, as shown inFIG. 7, the elevation angle is preferably adjusted by applying forces F1and F2to the upper and lower portions of the quartz etalon142. When plastic wires are used as shafts148and149, the elevation angle can be easily corrected by applying a force to, e.g., the lower portion if an excess force is applied to the upper portion.

This method performs adjustment by causing a semiconductor laser11to emit light, applying a dither signal to the quartz etalon142, measuring the output voltage from a synchronous detector145by a voltmeter14c, and measuring the wavelength of transmitted light in an optical filter transmission path13by a wave meter14d. In this adjusting method, the elevation angle need only be so adjusted that the synchronous detection output is 0 at the wavelength to be set. In this way, the adjusting method of this embodiment can exceptionally increase the wavelength accuracy with extremely simple adjustment.

As described above, the wavelength stabilizing apparatus of the above embodiment can sharply increase the signal-to-noise ratio of a signal obtained from the wavelength detection system, and greatly increase the accuracy of the wavelength to be stabilized. It is also possible to eliminate adjustment of the frequency of a dither signal, so the adjusting time can be shortened. Furthermore, angle adjustment for adjusting the wavelength characteristic of the quartz etalon142can be performed by operating the synchronous detection system after the etalon is fixed. Accordingly, the wavelength setting accuracy can be especially increased with a simple adjusting operation.

In each of the above embodiments, laser-beam emitted from the back of the semiconductor laser11is sent into the wavelength stabilizing apparatus14. However, the front laser-beam has the same wavelength as that of the rear laser-beam. Therefore, the same effect can be obtained by branching the laser-beam emitted from the front and sending the branched light into the wavelength stabilizing apparatus14.

Also, in each of the above embodiments, the shape of the quartz etalon142is a rectangular parallelepiped. However, the present invention is not limited to this shape. That is, the quartz etalon can take any shape as long as the etalon has reflecting film layers opposing each other. The point is that the effect of the present invention can be obtained by holding the central portion of the quartz etalon by a dither signal, and transmitting laser-beam through a portion which functions as a node of the mechanical resonance.