Multi-carrier optical transmission system, optical transmitter, and optical receiver

An optical transmitter transmits to an optical receiver a multi-carrier modulated signal light by driving a light source with a modulated signal modulated with a multi-carrier modulation scheme. The optical receiver monitors reception characteristic of any of subcarrier signals included in the modulated signal and transmits a monitor result to the optical transmitter. The optical transmitter controls drive conditions of the light source based on the monitor result received from the optical receiver.

FIELD

The embodiment discussed herein is related to a multi-carrier optical transmission system, an optical transmitter, and an optical receiver.

BACKGROUND

In recent years, with increasing transmission traffic, demand for larger capacities of optical transmission systems is increasingly on the rise. In short-distance transmission systems, not only larger capacities but also low-cost and simple configurations are demanded.

Thus, an application of a discrete multi-tone (DMT) modulation scheme to an optical transmission system has been discussed. The DMT modulation scheme is one of multi-carrier transmission technologies based on an orthogonal frequency division multiplexing (OFDM) technology and is used in a digital subscriber line (DSL) technology such as an asymmetric DSL (ADSL).

The DMT modulation scheme uses a “bit loading” which allocates bit numbers to each subcarrier in accordance with its transmission characteristic. Examples of the transmission characteristic may include a signal to noise ratio (SNR) and/or a bit error rate (BER). The transmission characteristic may also be referred to as a transmission condition. For example, more bits are allocated to a subcarrier with a high transmission characteristic than to a subcarrier with a low transmission characteristic. Thus, it is possible to improve a spectral efficiency.

When the DMT modulation scheme is installed in an optical transmission system, an electric/optic (E/O) converter using a direct modulation scheme for a semiconductor laser is applicable to an optical transmitter. Further, an optic/electric (O/E) converter using a light reception element such as a PD (photo detector or photo diode) is applicable to an optical receiver.

The semiconductor laser and the PD are general-purpose optical devices and thus are inexpensive. In the direct modulation scheme, a modulated signal is generated by modulating a drive current of the semiconductor laser that is a light source with transmission information. When compared with an external modulation scheme using an optical modulator in addition to a light source, the direct modulation scheme is difficult in a speedup but is possible to achieve downsizing and cost reduction because it is unnecessary to use phase information of a signal light. The semiconductor laser used in the direct modulation scheme may be referred to as a direct modulation laser (DML).

Therefore, by installing the DMT modulation scheme in an optical transmission system, it is possible to provide an optical transmission system available to improve a spectral efficiency (in other words, improve transmission capacities) and available to achieve downsizing and cost reduction.

As examples of optical transmission technology, there are technologies described in JP 11-127119 A and JP 6-303196 A.

For example, in a transmitting side, a transmission analog signal is multiplexed with a pilot signal which has a single frequency in a frequency band far apart from that of the transmission analog signal and has a fixed amplitude and the multiplexed signals are transmitted to an optical transmission line. Then, variations of a transmission gain in the optical transmission line are monitored by monitoring the pilot signal halfway through the optical transmission line and a gain of a variable gain amplifier provided on the optical transmission line is controlled so that the monitored value is constant.

As another example, an optical reception unit detects an instantaneous worst value of distortion of a reception signal to feedback the detected value to an optical transmission unit. The optical transmission unit determines a modulation level of a frequency multiplexed digital signal light to be transmitted and the number of frequency division multiplexing channels based on the received instantaneous worst value.

In an optical transmission system using the direct modulation scheme, a transmission characteristic changes depending on drive conditions (for example, the amplitude and a bias current of a drive current) of a semiconductor laser that is an example of a light source. Thus, it is preferable to optimize the drive conditions of the semiconductor laser to ensure transmission characteristics expected for an optical transmission system.

Further, unlike an NRZ (Non-Return-to-Zero) scheme, since the DMT modulation scheme changes a bit allocation (or multi-level modulation format) and/or transmission power of each subcarrier in accordance with the transmission characteristics, it is preferable to optimize the drive conditions of the semiconductor laser for each transmission characteristic.

In the technologies described above, however, no discussion has been made in an application of the DMT modulation scheme to an optical transmission system. Therefore, no discussion has also been made in optimizing drive conditions of the semiconductor laser in accordance with the transmission characteristics.

SUMMARY

An aspect of an optical transmission system may include an optical transmitter and an optical receiver. The optical transmitter may transmit a multi-carrier modulated signal light by driving a light source with a modulated signal modulated by using a multi-carrier modulation scheme. The optical receiver may receive the multi-carrier modulated signal light transmitted by the optical transmitter to demodulate the modulated signal. The optical receiver may monitor a reception characteristic of any of subcarrier signals included in the modulated signal and transmit a monitor result obtained by the monitoring to the optical transmitter. The optical transmitter may control a drive condition of the light source based on the monitor result received from the optical receiver.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment will be described with reference to the drawings. However, the embodiment described below is only by way of example and does not intend to exclude application of various modifications and technologies that are not described explicitly below. Further, various illustrative aspects described below may be carried out by appropriately combining such aspects. Incidentally, portions to which the same reference signs are given in the drawings used for the embodiment below represent the same or similar portions unless otherwise stated.

FIG. 1is a block diagram illustrating a configuration example of a multi-carrier optical transmission system applied with a DMT modulation scheme, according to an embodiment. A multi-carrier optical transmission system1illustrated inFIG. 1includes, for example, an optical transmitter10and an optical receiver30connected to the optical transmitter10via an optical transmission line50using an optical transmission medium such as an optical fiber. One or more of optical amplifiers may be provided on the optical transmission line50.

The optical transmitter10converts transmission data modulated by the DMT modulation scheme into a signal light by using the direct modulation scheme and transmits the obtained transmission modulated signal light to the optical transmission line50. The DMT modulation scheme is an example of the multi-carrier modulation scheme. The transmission modulated signal light may be referred to as a DMT modulated signal light. The DMT modulated signal light is an example of a multi-carrier modulated signal light.

The optical receiver30converts the DMT modulated signal light received from the optical transmission line50into an electric signal and performs a DMT-demodulation on the electric signal to obtain reception data.

Thus, the optical transmitter10may include, for example, a DMT modulator11, a direct modulation laser (DML)12, a DML driver13, and a laser drive controller14.

The DMT modulator11is an example of a multi-carrier modulator and applies a DMT-modulation to transmission data of an electric signal to generate a DMT modulated signal. Thus, the DMT modulator11may include, for example, a serial to parallel (S/P) converter111, a mapper112, an inverse fast Fourier transformer (IFFT)113, and a combiner114.

The S/P converter111performs an S/P-conversion on transmission data to generate parallel data whose numbers are according to the number of subcarriers and input the parallel data into the mapper112.

The mapper112maps the parallel data having digital bit sequences input from the S/P converter111to a symbol on a complex plane, which is also referred to as an IQ plane, for each subcarrier. The mapping may also be referred to as a “subcarrier modulation”. An exemplary arrangement of the subcarriers is illustrated inFIG. 2.FIG. 2illustrates a case where N (N is an integer equal to 2 or greater) subcarriers are set in a frequency domain. The number1to N represent a subcarrier number.

In the DMT modulation scheme, it is available to allocate more bits to a subcarrier having a high transmission characteristic than to a subcarrier having a low transmission characteristic for each symbol. As illustrated inFIG. 3, when the transmission characteristic becomes better with a decreasing subcarrier number of the subcarrier, it is available to allocate more bits to a subcarrier having a small subcarrier number than to a subcarrier having a large subcarrier number for each symbol.

The number of allocated bits may correspond to a multi-level of the subcarrier modulation. For example, when a quadrature phase shift keying (QPSK) is applied to the subcarrier modulation, the multi-level corresponds to 2. When a 2M-QAM (Quadrature Amplitude Modulation) is applied to the subcarrier modulation, the multi-valued level corresponds to M (M=4, 6, 8 and so on).

The IFFT113performs IFFT processing on signal, which is mapped to a symbol for each subcarrier by the mapper112in the frequency domain, to convert the signal into a time domain.

The combiner114combines signals in the time domain obtained by the IFFT113to generates a DMT modulated signal.

The DML driver13amplifies the DMT modulated signal obtained by the DMT modulator11(for example, by the combiner114) such that drive conditions (for example, the bias current and the amplitude) determined by the laser drive controller14are satisfied to generate a drive current for the DML12.

The DML12is an example of the light source and may be a semiconductor laser, for example. The DML12varies in its light emission power in accordance with the DMT modulated drive current given from the DML driver13to generate a DMT modulated signal light and output the generated DMT modulated signal light to the optical transmission line50.

The laser drive controller14controls the drive conditions of the DML12given by the DML driver13. The drive conditions of the DML12may also be referred to as “laser drive conditions”.FIG. 4illustrates an example of the relationship between the drive current of the DML12and output optical power of the DML12.FIGS. 5, 6, and 7illustrate examples of a secondary harmonic component characteristic, a frequency characteristic, and a relative intensity noise (RIN) characteristic with respect to the frequency when the drive amplitude is changed, respectively. These characteristics are examples of characteristics of the DML12. The RIN is obtained by normalizing fluctuations of light intensity caused by interference between a stimulated emission light and a spontaneous emission light in a laser light using average optical power.

As illustrated inFIG. 4, the output optical power of the DML12is varied by causing a drive current to vary in a certain amplitude in positive and negative directions relative to the bias current value. The amplitude of a drive current may be referred to as a “drive amplitude”. It is possible to operate the DML12in a linear region by setting the bias current value and the drive amplitude appropriately.

However, when the drive amplitude is too large, the DML12operates in a nonlinear region and, as illustrated inFIG. 5, since the secondary harmonic component increases in output optical power of the DML12, the transmission characteristic is deteriorated.

On the other hand, when the drive amplitude is too small, as illustrated inFIG. 6, the band of the frequency characteristic becomes narrower. For example, when frequency responses (dB) at the drive currents are 40 mA, 60 mA, 80 mA, and 100 mA as indicated by symbols A to D respectively are compared, the band in which the same frequency response as that when the drive current is the largest 100 mA is obtained becomes narrower with decreasing in the drive current.

Also, as illustrated inFIG. 7, the RIN increases with decreasing in the drive current and the transmission characteristic is deteriorated.FIG. 7illustrates the RINs (dB/Hz) with respect to frequencies when the drive currents are 15 mA, 25 mA, 35 mA, 45 mA, 55 mA, and 65 mA as indicated by symbols A to F, respectively.

Therefore, in order to ensure the transmission characteristic (for example, SNR, BER or the like) expected for the multi-carrier optical transmission system1, it is important to optimize drive conditions of the DML12. Further, since the multi-level and transmission power for each subcarrier are optimized in accordance with the transmission characteristic in the DMT modulation scheme, it is important to optimize the drive conditions of the DML12for each transmission characteristic.

Characteristics (hereinafter, may also be referred to as “laser characteristics”) of the DML12are affected by the RIN and the frequency response depending on the frequency, as described above. Thus, there is a subcarrier (or frequency) affected by the RIN and the frequency response more than any other subcarrier.

In the present embodiment, therefore, the reception characteristic (for example, SNR, BER or the like) of such subcarrier signal is monitored (or measured) by the optical receiver30(for example, a subcarrier monitor33described below), and drive conditions of the DML12are controlled based on the monitor result.

The monitor result can be transmitted (or fed back) to the optical transmitter10through an optical transmission line (omitted inFIG. 1) that transmits light in the opposite direction in the optical transmission line50. For example, the monitor result can be set to an optical supervisory channel (OSC) signal that is an example of monitor control signal light.

Therefore, the laser drive controller14illustrated inFIG. 1determines the drive conditions of the DML12based on the monitor result fed back from the optical receiver30. For example, the laser drive controller14may store a table141in which a combination (or correspondence) of an SC monitor result and laser drive conditions are set and determine the drive conditions based on the monitor result with reference to the table141.

The table141may also be referred to as an SC monitor result-laser drive conditions table141. The table141may be stored in a storage unit (not illustrated) such as a memory provided in the laser drive controller14. An exemplary content of the table141will be described below with reference toFIG. 11.

Next, the optical receiver30illustrated inFIG. 1may include, for example, a PD (photo detector or photo diode)31as an example of a light reception element, a DMT demodulator32, a subcarrier (SC) monitor33, and an SC monitor result transmitter34.

The PD31converts a DMT modulated signal light received from the optical transmission line50into an electric signal having an amplitude corresponding to the received light power.

The DMT demodulator32performs a DMT-demodulation on an electric signal obtained by the PD31to obtain reception data. Thus, the DMT demodulator32may include, for example, a divider321, a fast Fourier transformer (FFT)322, a de-mapper323, and a parallel to serial (P/S) converter324.

The divider321divides an electric signal input from the PD31into signals corresponding to the subcarriers to input the signals into the FFT322.

The FFT322performs FFT processing on each of the signals input from the divider321to convert the signals in the time domain into signals in a frequency domain.

The de-mapper323performs a subcarrier demodulation (or a DMT demodulation) on a received signal by identifying a reception symbol on the IQ plane from a signal in the frequency domain obtained by the FFT322and extracting (or de-mapping) bits mapped to the reception symbol.

The DMT-demodulated received signal for each subcarrier (or subcarrier signal) is P/S-converted by the P/S converter324and output as serial reception data.

The subcarrier monitor33monitors (or measures), for example, one or more of reception characteristics of the subcarrier signal input from the de-mapper323to the P/S converter324. Examples of reception characteristics may include the SNR and/or the BER.

The subcarrier signal to be input to the subcarrier monitor33may be fixed to a signal of a preset subcarrier number or may be changeable to a signal of a different subcarrier number in accordance with the settings. For example, one or more of subcarrier signals output from the de-mapper323may selectively be input to the subcarrier monitor33by providing a selector in an input stage of the subcarrier monitor33and setting the selector.

The subcarrier monitor result transmitter34transmits (or feedbacks) a reception characteristic (in other words, monitor result) of the subcarrier signal monitored (or measured) by the subcarrier monitor33to the laser drive controller14of the optical transmitter10. The subcarrier monitor result transmitter34may be implemented by using an OSC transmission unit which superimposes the monitor result on an OSC light transmitted to the optical transmitter10.

Next, a setting example of a subcarrier to be monitored (hereinafter, may be referred to as a “monitor subcarrier”) by the subcarrier monitor33will be described. The monitor subcarrier may be set to a subcarrier corresponding to a frequency that affects, as described above, characteristics of the DML12. A setting example of the monitor subcarrier is illustrated inFIGS. 8 and 9.

Graphs A to F illustrated on the left side ofFIG. 8illustrate examples of, similar toFIG. 7, the RIN characteristic with respect to the frequency, which is one of laser characteristics. Graphs A to C illustrated on the right side ofFIG. 8illustrate examples of the RIN characteristic for a drive current when the frequency is 1 GHz, 7 GHz, and 20 GHz, respectively.

As understood fromFIG. 8, since the frequency of 7 GHz corresponds to a frequency near a relaxation oscillation frequency of the DML12, variations of the RIN caused by a change of the drive current tend to be larger than other frequencies. Therefore, when the monitor subcarrier at the subcarrier monitor33is set to a subcarrier corresponding to 7 GHz, it is possible to sensitively monitor an influence of the RIN characteristic which is one of the laser characteristics on the transmission characteristic of the DMT modulated signal. The relaxation oscillation frequency may correspond to a threshold frequency for a response characteristic to a drive current. In a frequency region exceeding the threshold frequency, since a wavelength (or frequency) chirping occurs in output light of the DML12, a response characteristic with respect to a drive current deteriorates sharply.

Graphs A to D illustrated on the left side ofFIG. 9illustrate examples of, similar toFIG. 6, the frequency characteristic as one of laser characteristics. Graphs A to C illustrated on the right side ofFIG. 9illustrate examples of the frequency response characteristic for a drive current when the frequency is 2 GHz, 7 GHz, and 28 GHz, respectively.

As understood fromFIG. 9, changes of the frequency response caused by a change of the drive current tend to be larger when the frequency is 28 GHz than when the frequency is any other frequency. Therefore, among 2 GHz, 7 GHz, and 28 GHz, the monitor subcarrier may be set to a subcarrier corresponding to 28 GHz on a higher frequency side. Accordingly, it is possible to sensitively monitor an influence of the frequency characteristic (in other words, frequency response) which is one of the laser characteristics on the transmission characteristic of the DMT modulated signal.

In view of the above, as illustrated inFIG. 10, two subcarriers of 7 GHz and 28 GHz may be set as the monitor subcarriers in the present embodiment as a non-restrictive example and the subcarrier monitor33monitors the reception characteristic of each subcarrier.

The laser drive controller14recognizes an operating state of the DML12from a monitor result of each monitor subcarrier to control drive conditions of the DML12. For example, the laser drive controller14determines operation conditions of the DML12with reference to the table141in which the reception characteristic is associated with the drive conditions of the DML12, based on the reception characteristic of a subcarrier signal monitored by the subcarrier monitor33.

FIG. 11illustrates an example of the table141stored in a storage unit such as a memory provided in the laser drive controller14.FIG. 11illustrates a case where four entries are set to the table141. Each entry has a combination of the SNR, the bias current and the drive amplitude. The SNR is an example of the reception characteristic. The bias current and the drive amplitude are examples of drive conditions.

The first entry has data to set the bias current and the drive amplitude of the DML12to 60 mA and 40 mA, respectively, when the reception characteristics (for example, SNRs) of monitor carriers of 7 GHz and 28 GHz are 20 dB and 10 dB, respectively.

The second entry has data to set the bias current and the drive amplitude of the DML12to 80 mA and 40 mA, respectively, when SNRs of the respective monitor carriers are 20 dB and 12 dB.

The third entry has data to set the bias current and the drive amplitude of the DML12to 60 mA and 60 mA, respectively, when SNRs of the respective monitor carriers are 25 dB and 10 dB.

The fourth entry has data to set the bias current and the drive amplitude of the DML12to 80 mA and 60 mA, respectively, when SNRs of the respective monitor carriers are 25 dB and 12 dB.

The laser drive controller14may select one of the above four entries based on the reception characteristics of the monitor subcarriers and to set and control the DML driver13to the laser drive conditions indicated in the selected entry.

Instead of using the table141, the laser drive controller14may control laser drive conditions by, for example, calculating a formula or an approximation formula created from the content of the table141. However, the response speed of drive control can be speeded-up by using the table141.

Operation Example

Next, an example of the drive control for the DML12in the multi-carrier optical transmission system1will be described with reference to the flow chart illustrated inFIG. 12.

First, the monitor subcarrier number is set to the subcarrier monitor33and the laser drive controller14(Process P11) and initial values of drive conditions (for example, the bias current and the drive amplitude) of the DML12are set to the laser drive controller14(Process P12).

The optical transmitter10generates a DMT modulated signal by the DMT modulator11(Process P13) and amplifies the DMT modulated signal by the DML driver13to satisfy the drive conditions (the initial values) determined by the laser drive controller14to input the amplified DMT modulated signal into the DML12as a drive current. Thereby, the DMT modulated signal is converted into an optical signal (in other words, a DMT modulated signal light) by the DML12and is transmitted to the optical transmission line50(Process P14).

The optical receiver30receives the DMT modulated signal light from the optical transmission line50by the PD31, converts the DMT modulated signal light into an electric signal corresponding to the received light power, and demodulates the electric signal by the DMT demodulator32(Process P15). In this regard, the DMT demodulator32extracts a received signal corresponding to the monitor subcarrier number set in Process P11to input the extracted signal to the subcarrier monitor33(Process P16).

The subcarrier monitor33monitors the reception characteristic of the signal corresponding to the monitor subcarrier input from the DMT demodulator32(Process P17). The subcarrier monitor33determines whether the reception characteristic of the monitored subcarrier signal indicates a value better than a predetermined value (Process P18).

For example, when the reception characteristic is monitored (or measured) as the BER, the subcarrier monitor33determines whether the monitored BER is smaller than a threshold thereof. Meanwhile, when the reception characteristic is monitored (or measured) as the SNR, for example, the subcarrier monitor33determines whether the monitored SNR is larger than a threshold thereof.

As a result of the determination, when the reception characteristic of the monitored subcarrier signal is better than the predetermined value (YES in Process P18), there is no need to change drive conditions of the DML12. Thus, the subcarrier monitor33may terminate the process without providing the monitor result to the subcarrier monitor result transmitter34.

Meanwhile, when the reception characteristic of the monitored subcarrier signal is not better than the predetermined value (for example, the monitored BER is equal to or larger than the threshold, or the monitored SNR is equal to or smaller than the threshold) (NO in Process P18), the subcarrier monitor33provides the monitor result to the subcarrier monitor result transmitter34. The subcarrier monitor result transmitter34transmits the monitor result to the optical transmitter10and the optical transmitter10provides the received monitor result to the laser drive controller14(Process P19).

The laser drive controller14determines drive conditions of the DML12(Process P20) with reference to the table141illustrated inFIG. 11based on the reception characteristic of the provided signal corresponding to the monitor subcarrier to control the DML driver13such that the determined drive conditions are satisfied. Thereby, the drive conditions of the DML12are changed from the initial values thereof (Process P21). Therefore, the monitor result obtained by the optical receiver30is an example of control information to control drive conditions of the DML12in the optical transmitter10.

After the drive conditions are changed, the process returns to Process P13and a DMT modulated signal light is to transmitted from the optical transmitter10to the optical transmission line50again. Hereinafter, processes described in Process P13to Process P21are repeated until the reception characteristic of the monitor subcarrier indicates a value better than the threshold (until determined YES in Process P18).

According to the above embodiment, as described above, one reception characteristic (for example, the BER, the SNR or the like) of (one or more of) subcarrier signals included in a DMT modulated signal is monitored and drive conditions (for example, the bias current and the drive amplitude) of the DML12are controlled based on the monitor result.

Therefore, it is possible to optimize the drive conditions of the DML12and to suppress the deterioration of the transmission characteristic of the DMT modulated signal due to the laser characteristics such as the frequency characteristic (in other words, the frequency response) and the RIN characteristic.

Here, by setting the monitor subcarrier to a subcarrier that impacts on the transmission characteristic of the DMT modulated signal due to the laser characteristics than any other frequency, it is possible to sensitively monitor (or detect) variations of the transmission characteristic of the DMT modulated signal.

For example, by setting the monitor subcarrier to a subcarrier near the relaxation oscillation frequency (for example, 7 GHz), it is possible to sensitively monitor variations of the transmission characteristic due to the RIN characteristic of the DML12. Therefore, it is possible to optimize the drive conditions of the DML12reliably in accordance with variations of the transmission characteristic due to the RIN characteristic.

Also, by setting the monitor subcarrier to a subcarrier on the high-frequency side (for example, 28 GHz), it is also possible to monitor variations of the transmission characteristic due to the frequency response of the DML12. Therefore, it is possible to optimize drive conditions of the DML12reliably in accordance with variations of the transmission characteristic due to the frequency response.

Further, the laser drive controller14determines (or controls) the drive conditions based on the table141illustrated inFIG. 11in which the correspondence between the reception characteristic of the monitor subcarrier and the drive conditions of the DML12are defined. Therefore, it is possible to improve a response speed of the drive control of the DML12.

In the embodiment described above, as a non-restrictive example, two subcarriers of 7 GHz and 28 GHz are set as the monitor subcarriers but a single monitor subcarrier (for example, one of 7 GHz and 28 GHz) may be set. Further, three subcarriers or more may be set as the monitor subcarriers.

According to the technology described above, it is possible to suppress the deterioration of transmission characteristic by optimizing the drive conditions of the light source applied to the multi-carrier optical transmission system.