Abstract:
A cold-start wavelength-division-multiplexed (WDM) optical transmission system uses a synchronized etalon filter, DFB lasers and passive optical demultiplexers. The DFB lasers are tested under normal operating conditions and wavelength-selected for each channel. The wavelength selection is such that the laser operates within one half of the channel spacing from the standardized wavelength. For the cold-start operation, each laser is able to automatically find its operating wavelength without any prior knowledge of its operating conditions for the standardized wavelength. In addition, this system simultaneously adjusts the output power of each laser to a desired value. Also provided is a method for generating standardized frequencies for a wavelength-division-multiplexed optical transmission system, including the step of adjusting the incident angle of an etalon filter by employing two frequency-locked lasers to compensate for differences of the thickness and refractive index of the etalon filter, and the step of fixing the position of the etalon filter to permanently have the adjusted incident angle, the etalon filter having resonant frequencies respectively corresponding to standardized frequencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of Applicant&#39;s Ser. No. 08/964,568, filed in the U.S. Patent &amp; Trademark Office on Nov. 5, 1997, now abandoned. 
    
    
     CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 and §120 from an application for A COLD-START WAVELENGTH-DIVISION-MULTIPLEXED OPTICAL TRANSMISSION SYSTEM AND METHOD FOR GENERATING STANDARD WAVELENGTHS THEREFOR earlier filed in the Korean Industrial Property Office on May 7, 1997 and there duly assigned Serial No. 17558/1997. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a cold-start wavelength-division-multiplexed (WDM) optical transmission system and method for generating standard wavelengths used therefor. 
     2. Description of the Related Art 
     The WDM system is used to multiplex a plurality of transmitter lasers on a single optical fiber, considerably increasing the transmission capacity per optical fiber even if the lasers operate at low speed. Hence, it enables the transmission capacity to be increased without replacing the embedded single-mode fiber with a distributed transition optical fiber. However, WDM systems require each transmitter laser to operate at a specific wavelength to the end of the systems&#39; lifetime. Proposals to standardize these wavelengths are currently being reviewed by the International Telecommunication Union (ITU). Therefore, all the lasers to be used in WDM systems will be operated at the standard frequencies recommended by the ITU. 
     At present, practical WDM system use distributed feedback (DFB) lasers as the existing optical transmission system since various types of tunable lasers have yet to show the required reliability, manufacturability and mode stability. However, DFB lasers have limited wavelength tunability and are difficult to manufacture to precise standardized wavelengths. Thus, DFB lasers must be “wavelength-selected” for each channel, and then tuned by temperature to operate at the standardized wavelengths. Once the DFB laser is tuned either in the factory or in the field, it must maintain the wavelength over the expected system&#39;s lifetime. However, because of aging, the wavelengths of some DFB lasers may drift up to a few tens of gigahertz even when their operating conditions such as bias current and temperature remain constant. This drift would result in the reduction of system margin and cause crosstalk. This may complicate the installation and maintenance of WDM systems. In particular, it could be disastrous if the wavelength of any laser must be changed in the field by unskilled technicians in order to restore the impaired transmission quality. 
     The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited cold-start wavelength division multiplexed optical transmission system and method of the present invention: U.S. Pat. No. 5,589,969 to Taga et al., entitled Wavelength Division Multiplexed Optical Fiber Transmission Equipment, U.S. Pat. No. 5,606,573 to Tsang, entitled Method And Apparatus For Control Of Lasing Wavelength In Distributed Feedback Lasers, U.S. Pat. No. 5,469,288 to Onaka et al., entitled Optical Filter, Method Of Controlling Transmission Wavelength Thereof, And Optical Receiver Using The Method, U.S. Pat. No. 5,557,439 to Alexander et al., entitled Expandable Wavelength Division Multiplexed Optical Communications Systems, U.S. Pat. No. 5,566,018 to Lee et al., entitled Apparatus For Adjusting Channel Width Of Multi-Channel Fiber Amplifier Light Source, U.S. Pat. No. 5,387,992 to Miyazaki et al., entitled Optical Frequency Multiplex Carrier Control System, U.S. Pat. No. 5,305,134 to Tsushima et al., entitled Optical Frequency Division Multiplexing Transmitter And Optical Frequency Division Multiplexing Transmission Apparatus, U.S. Pat. No. 4,635,246 to Taylor et al., entitled Frequency/Multiplex System Using Injection Locking Of Multiple Laser Diodes, U.S. Pat. No. 4,930,855 to Clark et al., entitled Wavelength Multiplexing Of Lasers, U.S. Pat. No. 4,813,756 to Frenkel et al., entitled Etalon Filters For Optical Channel Selection In Wavelength Division Multiplexed Fiber Systems, U.S. Pat. No. 5,119,454 to McMahon, entitled Bulk Optic Wavelength Division Multiplexer, U.S. Pat. No. 5,283,845 to Ip, entitled Multi-Port Tunable Fiber Optic Filter, and U.S. Pat. No. 5,438,579 to Eda et al., entitled Wavelength Stabilizing Apparatus. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a WDM optical transmission system which enables each laser to automatically find its operating wavelength and adjust its output power. 
     It is another object of the present invention to provide a WDM optical transmission system which controls the transmitter lasers to maintain the stable operation at the standardized frequencies to the end of the system&#39;s lifetime. 
     It is still another object of the present invention to provide a WDM optical transmission system which prevents the reduction of the system margin resulting from the wavelength drift. 
     It is a further object of the present invention to provide a WDM optical transmission system which prevents the crosstalk resulting from the wavelength drift. 
     It is further another object of the present invention to provide a WDM optical transmission system which simplifies the installation and maintenance thereof. 
     It is further another object of the present invention to provide a WDM optical transmission system with a device for generating the standardized laser frequencies. 
     The inventive WDM system utilizes DFB lasers and passive optical demultiplexers. The DFB lasers are tested under normal operating conditions (e.g., biased at 40 mA at 25° C.) and are wavelength-selected for each channel. The criteria for the wavelength selection are to have the laser operating within one half of the channel spacing from the standardized wavelength. However, these lasers are not necessarily tuned to the standardized wavelengths in the factory or in the field. Thus, for the cold start operation, each laser is able to automatically find its operating wavelength without any prior knowledge of its operating conditions for the standardized wavelength. In addition, this system may simultaneously adjust the output power of each laser to a desired value. Of course these lasers must operate at the standard frequencies to the end of the system&#39;s lifetime. 
     According to one aspect of the present invention, a WDM optical transmission system comprises: a reference providing device for providing equally-spaced references at standardized frequencies, a plurality of DFB transmitter lasers for operating near the standardized frequencies for different channels, a passive optical multiplexer for multiplexing the outputs of the DFB lasers, a controller for controlling the DFB laser to automatically operate at the standardized frequencies to generate outputs of desired levels, and a passive optical demultiplexer for demultiplexing the WDM output of the passive optical multiplexer. 
     The reference providing device is achieved by employing a solid etalon filter. A part of the output of the passive multiplexer is delivered to the etalon filter to provide references at the standardized wavelengths. The remaining part is delivered to the demultiplexer. If the multiplexer has only one output, a 2×2 star coupler is added to generate two outputs. The controller includes a photo-detector for converting the optical output signal of the etalon filter into an electrical signal, a phase-sensitive detector for detecting the first derivative signal from the etalon filter, and a control circuit for changing the operating temperature of the DFB lasers so as to conform their frequencies to the resonant frequencies of the etalon filter according to the first derivative signal obtained from the phase-sensitive detector. 
     If the output signal of the phase-sensitive detector is zero and has negative slope, the frequency of the laser is considered to conform to the corresponding resonant frequency of the etalon filter, since the phase-sensitive detector receives the first derivative signal of the etalon filter as the input according to the laser frequency. The controller controls the phases sensitive detector to generate a zero output with a negative slope. In addition, a current control circuit consisting of a proportional amplifier and an integrator is provided to precisely adjust the bias current of each laser by processing the first derivative signal, so that the laser frequency is locked to the desired resonant peak when the laser frequency approaches the corresponding resonant frequency. 
     The present invention will now be described more specifically with reference to the drawings attached only by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a graph illustrating the transmission characteristics of the etalon filter to provide references at the standardized wavelength; 
     FIG. 2 is a schematic diagram illustrating a cold-start WDM optical transmission system according to the present invention; 
     FIG. 3 is a graph illustrating the first derivative signal of the synchronized etalon filter obtained from the phase-sensitive detector and the transmission characteristics of the filter according to the present invention; 
     FIG. 4 is a graph illustrating the spectrums of the transmitter lasers measured before the cold start according to the present invention; 
     FIG. 5 is a graph illustrating the spectrums of the transmitter lasers measured after the cold start according to the present invention; 
     FIG. 6 shows an etalon filter fixed to a metal support, in accordance with the principles of the present invention; and 
     FIG. 7 illustrates a device for adjusting the resonance frequency of an etalon filter, in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The etalon filter used in the present invention should provide resonant frequencies conforming to the standardized frequencies recommended by the ITU. The “m”th resonant frequency v m  is given by the following formula: 
     
       
           V   m   =mL=mC/ (2 nI  Cos θ)  Formula 1. 
       
     
     where m represent the mode number, L is the free spectral range (FSR), C is the speed of light, n is the refractive index, I is the thickness of the filter, and θ is the incident angle of the optical signal. 
     A solid-type etalon filter is preferably used because of this simple structure and mechanical stability. However, it is very hard to readily manufacture the etalon filter to meet the requirement of thickness and refractive index. Hence, this requirement may be met by adjusting the incident angle. The incident angle is adjusted by using two master reference lasers. For example, the incident angle θ is adjusted to make the etalon filter have k+1 resonant frequencies between the upper and lower resonant frequency limits v m  and v m−k  conforming to the master frequencies v 1  and v 2 . The adjustment of the incident angle θ changes the mode number m and FSR L to satisfy the requirement of thickness and refractive index. 
     The synchronized etalon filter is used to provide the standardized frequencies, and such an etalon filter is disclosed in an article entitled as “Synchronized Etalon Filters for Standardizing WDM Transmitter Laser Wavelengths” by Y. C. Chung and L. W. Shulz,  IEEE PHOTONICS TECHNOLOGY LETTERS,  Vol. 5, No. 2, pp. 186-189, February 1993 and also disclosed in U.S. Pat. No. 5,434,877 entitled “Synchronized Etalon Filters” granted to Y. C. Chung on Jul. 18, 1995. The etalon filter has the incident angle adjusted by using two master frequencies of the lasers locked to Kr transitions to compensate for the differences of thickness and refractive index. Thus, the resonant frequency of the filter is fixed at a value, e.g., 195.575±n×0.1034 Thz. 
     Thus, the physical characteristics of the atom determine the resonant frequency of the etalon filter. In this case, if the resonant frequency of the filter does not conform to the standardized frequency recommended by the ITU, this method has no value. Hence, in the present invention, a commercial wavemeter is used to lock the frequency of the laser. 
     Since the ITU has not made its final recommendation yet, we assume that the standardized wavelengths for WDM systems are 193.686±n×0.100 THz, where n represents the channel number. Solid etalons are constructed by coating both sides of 1.04 mm-thick polished fused-silica glasses with multiple layers of TiO 2 /SiO 2 . 
     The etalons are calibrated by using two master reference lasers. One laser is frequency-locked to the Kr 1s 2 -2p 8  transition at 1.54782 μm (193.686 THz). The other laser is frequency-locked at 1.56072 μm (192.086 THz) using a wavemeter. This makes the channel spacing exactly 100 GHz. The resonant frequencies of the etalon were monitored directly by using a broadband light source and an optical spectrum analyzer. In this way, 17 channels are established between these two absolute references. The incident angle was adjusted to match the (m)th and (m+16)th modes of the etalon to the absolute references at 1.54782 μm and 1.56072 μm, respectively. In this case, the mode number “m” is 1936 and FSR 100 GHz. 
     The synchronization of the etalon  50  shall now be described in more detail. The FIG. 7 illustrates a device for adjusting the resonance frequency of the etalon filter  50 . Here, the wavemeter  200  is used for stabilizing the two lasers  210  and  220 . The stabilized lasers  210  and  220  are used as absolute references for synchronization of the etalon filter  50 . In fact, it would be possible to make an etalon which provides any set of equally-spaced references by using both master reference lasers frequency-locked to any desired wavelengths with a wavemeter. In this invention, we established seventeen channels between these two absolute references. To examine the resonance mode of the etalon filter  50 , a broadband light source  230  and optical spectrum analyzer  240  are used. For synchronization of the etalon  50 , firstly we can adjust the incident angle of the etalon filter  50  so that there happen seventeen resonance modes between the two reference frequencies by using the optical spectrum analyzer  240 . Secondly, we can make the output of laser #2 ( 220 ) having passed the etalon to have its maximum value by making the “m”-th resonance frequency equal to the reference frequency of laser #2 ( 220 ). In this case, the output of the laser having passed the etalon varies according to the value “m”. Maintaining the above conditions, namely “seventeen resonance modes between the absolute references” and “the maximum value of the output of the reference laser #2 ( 220 )”, we can adjust the incident angle of the etalon  50  precisely for the output of the reference laser having passed the etalon  50  to have its maximum value. 
     The etalon filters mounted on a support made of a metal such as Duralumin, whose adjustment part is oxidized with an oxidizing agent such as hydrochloric acid to permanently fix the desired incident angle after adjustment. The method of fixing the etalon filter  50  to a support shall now be described in more detail. There needs to be a very precise adjustment of an incident angle to make the resonance frequency of the etalon  50  equal to the reference (or standard) frequency. For this purpose, the module is embodied as shown in FIG.  6 . Firstly, the etalon filter  50  is fixed to the filter mount  250  made of metal and two lenses  260  are used for incidenting optical signal into the filter mount  250  having the structure shown in FIG.  6 . At this time, the metal support is used for fixing the filter mount between the two lenses. Accordingly, we can adjust the incident angle of the etalon filter precisely by rotating the filter mount  250  fixed on the metal support  270 . We can adjust the incident angle of the etalon filter  50  to make the resonance frequency of the etalon  50  equal to the reference (or standard) frequency and we can fix the incident angle permanently by fixing the filter mount  250  on the metal support  270  with an oxidized agent. 
     FIG. 1 shows the transmission characteristics (optical spectrums) of the etalon filter when the incident angle was adjusted to satisfy the above conditions. The two highest peaks represent the absolute references obtained from the frequency-locked lasers. The result shows that this synchronized etalon filter provides references at the standardized wavelengths of 193.686±n×0.100 THz. In fact, it would be possible to make an etalon filter which provides any set of equally-spaced references by using both master reference lasers frequency-locked to any desired wave lengths recommended by the ITU with wavemeters. 
     There is shown in FIG. 2 a four-channel cold start WDM system. Whose standardized wavelengths were selected to be 1547.82±n×0.8 nm (193.686±n×0.100 Thz) where n is the channel number. 
     FIG. 3 shows a transmission curve of the synchronized etalon filter. The dotted line represents the first derivative signal of the lasers to these resonance peaks (for example, within the region A in FIG. 3) under normal operating conditions could be “wavelength-selected” for their corresponding channels. In this case, four lasers were selected operating at 1552.97, 1553.61, 1554.61, and 1555.24 nm, at 40 mA and 25° C. Their corresponding channel numbers, n, were from 6 to 9. 
     FIG. 4 shows the optical spectrum of four wavelength-selected DFB lasers at the normal operating conditions, i.e., every laser being biased at 40 mA at 25° C. before the cold-start control. Although each laser was operating within one half of the channel spacing (±0.4 nm) from the corresponding standardized wavelength, both the channel spacing and the output powers were quite irregular. In order to eliminate such irregularity, the cold start WDM optical transmission system was operated. The DFB lasers were modulated simultaneously at 5 to 8 kHz, respectively, by adding a small sinusoidal current to the injection current. This was necessary not only to obtain the first-derivative signal, but also to identify each channel. This also reduced stimulated Brillouin scattering. The outputs of these laser were coupled into a 4×4 star coupler  20 , having one output fiber which was sent to the receiver via a transmission fiber and having another output fiber which was directed to the synchronized etalon filter  50 . The 4×4 star coupler served the multiplexer. If the multiplexer has only a signal output as the integrated optic waveguide grating, we may obtain two outputs necessary for the controlling and transmission by adding a 2×2 star coupler. The signal from the etalon filter  50  was sent to a PIN photodiode  60  that was connected to four phase-sensitive detectors. The control mechanism of the WDM optical transmission system is achieved by the PIN photodiode  60 , phase-sensitive detectors  70 , proportional amplifier  80 , integrator  90 , switch  110  and controller  100 . 
     Initially, in the cold-start WDM optical transmission system, the control loop set the bias current and the operating temperature of each laser to be 40 mÅ and 20° C., which is five degrees lower than its normal operating temperature. This was to set the wavelength of each laser in the region B in FIG. 3 with the system&#39;s channel interval of 100 GHz. The temperature-tuning coefficients of DFB lasers were quite uniform at 10 GHz/° C. 
     The controller  100  is controlled so as to increase progressively the operating temperature of each laser so as to move the frequency to the region where the output value of the phase-sensitive detector becomes positive. This was to prevent any laser from operating in the region D in FIG. 3 at the initial conditions to be locked in the wrong channel. Then, the output power of each laser was automatically adjusted to 0 dBm by using the optical output sensing signal Pwr of the photodiode built in the laser package. Then, the operating temperature of each laser was increased progressively until the laser was confirmed to be operating in the region C in FIG.  3 . 
     The control loop recognized that the laser was operating in the region C when the first derivative signal decreased as the operating temperature increased. Once the laser was confirmed to be operating in the region C, the controller  100  automatically turned on the switch  110  to activate the current control circuit consisting of the proportional amplifier  80  and integrator  90 . This, in turn, adjusted the bias current of each laser to lock the laser frequency to the desired resonant peak. However, as the bias current adjustment also changed the output power of the laser, the output power was readjusted to 0 dBm by changing the temperature. 
     FIG. 5 shows the optical spectrum of four DFB lasers obtained by this procedure. The channel spacing and output power were measured to be 100±1 Ghz and 0±0.1 dBm, respectively. There was no human intervention after turning in the system&#39;s power supply. The error-free transmission (BER&lt;10 −9 ) was achieved within 20 s with the receiver sensitivity of −32 dBm at 2.5 Gb/s. The long term stability was assured by the continuous monitoring and adjustment of the laser wavelength and output power. In addition, if the system is stopped owing to an interruption of the power, etc., the operating conditions of the lasers stored in a memory may be used to restore the system. Hence, the cold start needs to be restarted only when the transmitter lasers are replaced. 
     In conclusion, the inventive cold-start WDM optical transmission system can start data communication without any manual adjustment of laser wavelengths or optical powers. This system is advantageous because there is no need to precisely adjust the wavelengths of WDM lasers in the factory or in the field, thus simplifying the installation and maintenance of the WDM systems. In addition, the continuous adjustment of each laser to operate at its standardized wavelength improves the reliability of WDM systems. Finally, use of cold-start laser control may reduce the cost of the DFB lasers usable in WDM systems by allowing a relaxed wavelength-selection procedure. 
     Although the present invention has been described in connection with the specific embodiments and the accompanying drawings, it will be readily apparent to those skilled in the art that various modifications and changes thereof may be made without departing from the gist of the present invention.