Method and apparatus for controlling bias point of optical transmitter

An optical transmitter includes: a modulating unit that modulates an optical signal based on an electric signal; a first detecting unit that detects a first variation width of a maximum output of the modulated optical signal; a second detecting unit that detects a second variation width of a minimum output of the modulated optical signal; a comparing unit that performs a comparison of the first variation width and the second variation width; and an adjusting unit that adjusts a bias potential of the electric signal based on a result of the comparison.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-307420, filed on Oct. 21, 2005, 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 technology for controlling the bias point of an optical transmitter automatically.

2. Description of the Related Art

A conventional optical transmitter for an optical communication system uses a modulation system, such as a direct modulation system, an internal modulation system, and an external modulation system, which is suitable for a type of a light source and a communication speed. In recent years, the external modulation system having a small variation in the wavelength of an optical signal (that is, a chirping) regardless of a transmission speed calls attention, and has been widely used.

FIG. 8is a block diagram of a conventional optical transmitter using the external modulation system. As shown inFIG. 8, an optical transmitter800includes a light source810, an external modulator820, a branch unit830, a photoelectric converter840, a feedback unit850, an oscillator860, a pulse driver870, a synchronous detector880, and a bias controller890.

The light source810generates unmodulated light by a light-emitting element, and outputs the unmodulated light to the external modulator820. The external modulator820modulates the unmodulated light according to electric signals input from the pulse driver870and the bias controller890, and outputs the modulated light to the branch unit830as an optical signal. The branch unit830branches the optical signal into two parts at a predetermined rate, for example, 9:1. The main part is input to an optical transmission path, while the other part is input to the photoelectric converter840.

The photoelectric converter840converts the optical signal input from the branch unit830into an electric signal, and outputs the electric signal-to the feedback unit850. The feedback unit850includes a filter851and an amplifier852. Only the low-frequency component of the electric signal from the photoelectric converter840passes through the filter851to the amplifier852, which amplifies the low-frequency component and outputs the amplified low-frequency component to the synchronous detector880.

A reference low-frequency signal is generated by the oscillator860, and output to the pulse driver870and the synchronous detector880. Transmission data is input to the pulse driver870as an input signal, superimposed on the reference low-frequency signal from the oscillator860, and output to the external modulator820as a modulation signal.

The synchronous detector880compares the low-frequency component of the electric signal from the amplifier852of the feedback unit850with the reference low-frequency signal from the oscillator860, and outputs a signal corresponding to a phase difference to the bias controller890. The bias controller890adjusts the potential of a bias signal (a bias point) to be input to the external modulator820based on the signal input from the synchronous detector880.

As described above, a modulation signal from the pulse driver870and the bias signal from the bias controller890are input to the external modulator820. The light transmission factor of the external modulator820varies according to the potential of the bias signal. The transmission factor is represented by an extinction characteristics curve specific to each kind of external modulators and each element of the external modulators. In other words, even when a modulation signal of the same amplitude is input, the amplitude of the optical signal output from the external modulator820varies greatly depending on the setting of the potential of the bias signal (the bias point). Therefore, in the block diagram of the optical transmitter800, the optical signal is fed back to the external modulator820through the branch unit830to maximize the amplitude of the optical signal. Such a technique is disclosed in, for example, Japanese Patent Application Laid-Open No. H10-123471 and Japanese Patent No. 3333133.

However, according to the optical transmitters described in the above documents, the amplitude and the bias point need to be set while monitoring the waveform each time when an optical signal is transmitted, which is troublesome for the user.

Furthermore, the conventional optical transmitter cannot adjust the bias point appropriately when the extinction characteristics of the optical modulator change greatly according to the usage environment or the usage time. Therefore, the amplitude of the modulation signal needs to be set large to achieve the extinction ratio required for the optical transmission, thereby increasing the power consumption.

SUMMARY OF THE INVENTION

An optical transmitter according to an aspect of the present invention includes: a modulating unit that modulates an optical signal based on an electric signal input to the modulating unit and outputs a modulated optical signal; a first detecting unit that detects a first variation width of a maximum output of the modulated optical signal; a second detecting unit that detects a second variation width of a minimum output of the modulated optical signal; a comparing unit that performs a comparison of the first variation width and the second variation width; and an adjusting unit that adjusts a bias potential of the electric signal based on a result of the comparison.

A method for an optical transmitter according to another aspect of the present invention includes: modulating an optical signal based on an electric signal and outputs a modulated optical signal; detecting a first variation width of a maximum output of the modulated optical signal; detecting a second variation width of a minimum output of the modulated optical signal; performing a comparison of the first variation width and the second variation width; and adjusting a bias potential of the electric signal based on a result of the comparison.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a block diagram of an optical transmitter according to the present invention. An optical transmitter100includes a light source110, an external modulator120, a branch unit130, a photoelectric converter140, a feedback unit150, a superimposed-signal source160, a pulse driver170, and a signal combining unit180.

The light source110generates unmodulated light by a light-emitting element, and outputs the unmodulated light to the external modulator120. The external modulator120modulates the unmodulated light according to an electric signal input from the signal combining unit180, and outputs the modulated light to the branch unit130as an optical signal. The light transmission factor of the external modulator120varies according to the electric signal input from the signal combining unit180. The external modulator120is, for example, a lithium niobate (LN) modulator with a substrate of lithium niobate (LiNbO3), or an electro-absorption (EA) modulator.

FIG. 2is a graph for explaining the principle of a bias point control according to the present invention. A horizontal axis of a graph210represents a voltage VEA (V) of an electric signal input to the external modulator120, and a vertical axis of the graph210represents optical output power of an optical signal output from the external modulator120. A curve211represents extinction characteristics of the external modulator120when the external modulator120is the EA modulator. A horizontal axis of a graph220represents the voltage VEA (V) of the electric signal input to the external modulator120, and a vertical axis of the graph220represents a slope of the curve211.

On the other hand, a curve221in the graph220represents the slope of the curve211, in other words, the absolute value of the quotient of □optical output power by □voltage VEA. An end □VH of the curve221is proportional to the variation width VH of the optical signal, and an end □VL of the curve221is proportional to the variation width VL of the optical signal. Therefore, the bias point can be adjusted at the center of the curve221, where the slope of the curve211becomes maximum, by detecting the variation widths VH and VL and by determining ends □VH and □VL based on the detected variation widths VH and VL. The optical transmitter100detects the variation widths VH and VL, performs a predetermined calculation on the variation widths VH and VL, respectively, compares the results of the calculation, and adjusts the bias point.

Referring back toFIG. 1, the external modulator120outputs an optical signal to the branch unit130. The branch unit130branches the optical signal into two parts at a predetermined rate, for example, 9:1. The main part is input to an optical transmission path, while the other part is input to the photoelectric converter140. The photoelectric converter140converts the optical signal input from the branch unit130into an electric signal, and outputs the electric signal to the feedback unit150.

The feedback unit150includes an H-side amplitude detector151, an L-side amplitude detector152, a first calculator153, a second calculator154, a comparator155, and a bias driver156. The H-side amplitude detector151detects the variation width VH of the H-side amplitude of the electric signal input from the photoelectric converter140. The L-side amplitude detector152detects the variation width VL of the L-side amplitude of the electric signal input from the photoelectric converter140.

The first calculator153performs a predetermined calculation on the variation width VH detected by the H-side amplitude detector151(for example, the multiplication of the variation width VH by n), and outputs the result to the comparator155. The second calculator154performs a predetermined calculation on the variation width VL detected by the L-side amplitude detector152(for example, the multiplication of the variation width VL by n), and outputs the result to the comparator155. Alternatively, the first calculator153and the second calculator154can output the detected variation widths as it is. The type of calculation is set in advance depending on the usage mode, as described in detail later.

The comparator155compares the results, and outputs the result of the comparison to the bias driver156. The bias driver156outputs, to the signal combining unit180, a bias signal corresponding to a bias point adjusted based on the comparison result.

A superimposed signal is generated by the superimposed-signal source160, and output to the signal combining unit180. Transmission data is input to the pulse driver170as an input signal, and output to the signal combining unit180. The signal combining unit180superimposes the input signal from the pulse driver170on the superimposed signal from the superimposed-signal source160, thereby generating a modulation signal. The signal combining unit180further combines the modulation signal with the bias signal input from the bias driver156, and outputs the combined signal to the external modulator120.

The optical transmitter100can also include a third calculator190between the photoelectric converter140and the feedback unit150. The third calculator190calculates the logarithm of the electric signal input from the photoelectric converter140, and outputs the logarithm to the feedback unit150. The configuration including the third calculator190is effective when the logarithm of the extinction characteristics of the external modulator120has the waveform as shown by the curve221.

A procedure of a bias point control according to a first embodiment of the present invention is explained next with reference toFIG. 3. The bias point control according to the first embodiment is the most basic one, which can be widely applied to modulators having point-symmetric extinction characteristics as shown inFIG. 2. As shown inFIG. 3, the bias driver156sets an EA bias as an initial bias point (step S301). A superimposed signal from the superimposed-signal source160and a modulation pulse from the pulse driver170are input to the signal combining unit180(step S302). The light source110inputs light to the external modulator120(step S303). The photoelectric converter140converts the light output from the external modulator120into an electric signal (step S304).

The H-side amplitude detector151detects the variation width VH based on the H-side amplitude of the electric signal, and sets the detected VH to a comparison value VLH (step S305). The L-side amplitude detector152detects the variation width VL based on the L-side amplitude of the electric signal, and sets the detected VL to a comparison value VL0(step S306).

Thereafter, the comparator155determines whether the comparison value VLH and the comparison value VL0are equal (step S307). When the comparison value VLH is equal to the comparison value VL0(step S307: Yes), the current setting of the bias is held (step S308), and the process proceeds to step S312. When the comparison value VLH is not equal to the comparison value VL0(step S307: No), it is determined whether the comparison value VLH is larger than the comparison value VL0(step S309).

When the comparison value VLH is larger than the comparison value VL0(step S309: Yes), the bias driver156increases the bias point (in other words, shift the bias potential of the modulation signal212shown inFIG. 2to the left side) (step S310), and the process proceeds to step S312. When the comparison value VLH is smaller than the comparison value VL0(step S309: No), the bias driver156decreases the bias point (in other words, shift the bias potential of the modulation signal212shown inFIG. 2to the right side) (step S311), and the process proceeds to step S312.

Then, it is determined whether the control should be ended (step S312). When the control is to be continued (step S312: No), the series of process from step S304to S312is repeated. When the control is to be ended (step S312: Yes), the series of process ends.

A procedure of a bias point control according to a second embodiment of the present invention is explained next with reference toFIGS. 4 and 5. The bias point control can be widely applied to modulators having different extinction characteristics from that shown inFIG. 2, that is, point-asymmetrical extinction characteristics shown inFIG. 4. A horizontal axis of a graph410represents a voltage VEA (V) of an electric signal input to the external modulator120, and a vertical axis of the graph410represents optical output power of an optical signal output from the external modulator120. A curve411represents extinction characteristics of the external modulator120. A horizontal axis of a graph420represents the voltage VEA (V) of the electric signal input to the external modulator120, and a vertical axis of the graph420represents a slope of the curve411.

When a modulation signal412is input to the external modulator120, the external modulator120outputs an optical signal413. The amplitude of the optical signal413corresponds to the amplitude of the modulation signal412around the bias signal. A variation width VH represents the variation width of amplitude at the H side of the optical signal413, and a variation width VL represents the variation width of amplitude at the L side of the optical signal413. On the other hand, a curve421in the graph420represents the slope of the curve411. An end DVH of the curve421is proportional to the variation width VH of the optical signal, and an end □VL of the curve421is proportional to the variation width VL of the optical signal.

The curve211shown inFIG. 2represents the extinction characteristics of a general EA modulator, which is substantially point symmetrical with respect to the point where its slope reaches the maximum. On the other hand, the extinction characteristics of the external modulator120used in the second embodiment is not point symmetrical as shown inFIG. 4. Specifically, as shown by the curve421, the slope of extinction characteristics is small at the H side, and large at the L side.

In the second embodiment, the bias point cannot be adjusted to an appropriate value (that is, a voltage maximizing the slope of the extinction characteristics) by keeping the end □VL equal to the end DVH as in the first embodiment. Therefore, in the second embodiment, the bias point is adjusted so that n times the end □VL of the curve421becomes equal to the end □VH. By setting the value of n appropriately, the bias point can be shifted to the position where the slope of the curve421reaches the maximum. Therefore, in the second embodiment, either the variation width VH detected by the H-side amplitude detector151or the variation width VL detected by the L-side amplitude detector152is multiplied by n, and the product is set as the comparison value.

FIG. 5is a flowchart of the bias point control according to the second embodiment. The bias driver156sets the EA bias as the initial bias point (step S501). A superimposed signal from the superimposed-signal source160and a modulation pulse from the pulse driver170are input to the signal combining unit180(step S502). The light source110inputs light to the external modulator120(step S503). The photoelectric converter140converts the light output from the external modulator120into an electric signal (step S504).

The variation width VH is detected by the H-side amplitude detector151based on the H-side amplitude of the electric signal, and the detected VH is set to the comparison value VLH (step S505). The variation width VL is detected by the L-side amplitude detector152based on the L-side amplitude of the electric signal, multiplied by n by the second calculator154, and the product is set to the comparison value VL0(step S506).

Thereafter, the comparator155determines whether the comparison value VLH and the comparison value VL0are equal (step S507). When the comparison value VLH is equal to the comparison value VL0(step S507: Yes), the current setting of the bias is held (step S508), and the process proceeds to step S512. When the comparison value VLH is not equal to the comparison value VL0(step S507: No), it is determined whether the comparison value VLH is larger than the comparison value VL0(step S509).

When the comparison value VLH is larger than the comparison value VL0(step S509: Yes), the bias driver156increases the bias point (step S510), and the process proceeds to step S512. When the comparison value VLH is smaller than the comparison value VL0(step S509: No), the bias driver156decreases the point (step S511), and the process proceeds to step S512.

Then, it is determined whether the control should be ended (step S512). When the control is to be continued (step S512: No), the series of process from step S504to S512is repeated. When the control is to be ended (step S512: Yes), the series of process ends.

The bias point control explained above assumes that the external modulator120has the extinction characteristics as shown inFIG. 4, in which the end □VH of the curve421is larger than the end DVL. However, when the external modulator120has extinction characteristics in which the end □VH is smaller than the end □VL, the variation width VH is multiplied by n and set to the comparison value VLH at step S505, and the variation width VL is set to the comparison value VL0as it is at step S506.

A procedure of a bias point control according to a third embodiment of the present invention is explained next with reference toFIG. 6. The bias point control can be widely applied to modulators having point-symmetric extinction characteristics as shown inFIG. 2, to minimize the sum of the variation widths VH and VL.

FIG. 6is a flowchart of the bias control according to the third embodiment. A reference potential V2for the comparator155is set to 0, and a variable S is set to 1 (step S601). The bias driver156sets the EA bias as the initial bias point (step S602). A superimposed signal from the superimposed-signal source160and a modulation pulse from the pulse driver170are input to the signal combining unit180(step S603). The light source110inputs light to the external modulator120(step S604). The photoelectric converter140converts the light output from the external modulator120into an electric signal (step S605).

The H-side amplitude detector151detects the variation width VH based on the H-side amplitude of the electric signal, and sets the detected VH to the comparison value VLH (step S606). The L-side amplitude detector152detects the variation width VL based on the L-side amplitude of the electric signal, and sets the detected VL to the comparison value VL0(step S607). VL0(step S607).

Thereafter, the sum of the comparison values VLH and VL0is set to a variable V1(step5608). When the value of variable V1is equal to or larger than that of the variable V2(step S609: Yes), the variable S is set to −S (step S611). When the value of variable V1is smaller than that of the variable V2(step S609: No), the variable S is set to S (step S610).

Then, it is determined whether the value of variable S is larger than 0 (step S612). When the value of variable S is larger than 0 (step S612: Yes), the bias driver156decreases the bias point (step S613). When the value of variable S is not larger than 0 (step S612: No), the bias driver156increases the bias point (step S614).

Then, the value of variable V2is set to the variable V1(step S615), and it is determined whether the control should be ended (step S616). When the control is to be continued (step S616: No), the series of process from step S605to S616is repeated. When the control is to be ended (step S616: Yes), the series of process ends.

A procedure of a bias point control according to a fourth embodiment of the present invention is explained next with reference toFIG. 7. The bias point control is for the optical transmitter100with the third calculator190.FIG. 7is a flowchart of the bias point control according to the fourth embodiment. The bias driver156sets the EA bias as the initial bias point (step S701). A superimposed signal from the superimposed-signal source160and a modulation pulse from the pulse driver170are input to the signal combining unit180(step S702). The light source110inputs light to the external modulator120(step S703). The photoelectric converter140converts the light output from the external modulator120into an electric signal (step S704).

The third calculator190calculates the logarithm V0of the output Vi of the electric signal, and outputs the logarithm V0to the feedback unit150(step S705).

Thereafter, the H-side amplitude detector151detects the variation width VH based on the logarithm V0and sets the detected VH to the comparison value VLH (step S705). The L-side amplitude detector152detects the variation width VL based pm the logarithm V0,7570and sets the detected VL to the comparison value VL0(step S707).

Thereafter, the comparator155determines whether the comparison value VLH and the comparison value VL0are equal (step S708). When the comparison value VLH is equal to the comparison value VL0(step S708: Yes), the current setting of the bias is held (step S709), and the process proceeds to step S713. When the comparison value VLH is not equal to the comparison value VL0(step S708: No), it is determined whether the comparison value VLH is larger than the comparison value VL0(step S710).

When the comparison value VLH is larger than the comparison value VL0(step S710: Yes), the bias driver156increases the bias point (step S711), and the process proceeds to step S713. When the comparison value VLH is smaller than the comparison value VL0(step S710: No), the bias driver156decreases the bias point (step S712), and the process proceeds to step S713.

Then, it is determined whether the control should be ended (step S713). When the control is to be continued (step S713: No), the series of process from step S704to S713is repeated. When the control is to be ended (step S713: Yes), the series of process ends.

As explained above, according to the present invention, the bias point can be adjusted automatically, and an optimum light transmission factor can be maintained.

The optical transmitter100according to the present invention detects the variation widths VH and VL based on an optical signal each time when the optical signal is output. Therefore, the bias point can be adjusted to a suitable point according to a change of extinction characteristics due to temperature. Consequently, the optical transmitter100, which is compact and does not require an automatic temperature control (ATC) circuit, can be provided at low cost.