Wavelength monitor, wavelength lockable laser diode and method for locking emission wavelength of laser diode

Abstract

A wavelength monitor monolithically integrated with a tunable LD is disclosed. The wavelength monitor includes at least two filters, each having a periodic transmission spectrum but a period between nearest neighbor periods is different from the other. A transmittance of the first filter and another transmittance of the second filter at a grid wavelength attributed to the WDM system forms a combination which is specific to the grid wavelength bur different from other combinations at other grid wavelengths.

What is claimed is: 
     
         1 . A wavelength monitor integrated with a tunable LD to form a wavelength lockable LD, comprising:
 a first optical filter to transmit light generated by the tunable LD, the first optical filter having a first transmittance spectrum periodically varying in a wavelength range of a WDM system; and   a second optical filter to transmit light generated by the tunable LD, the second optical filter having a second transmittance spectrum periodically varying in the wavelength range of the WDM system,   wherein the first transmittance spectrum and the second transmittance spectrum at a grid wavelength of the WDM system have a combination in respective transmittances specific to the grid wavelength different from combinations of respective transmittances at other grid wavelengths of the WDM system.   
     
     
         2 . The wavelength monitor of  claim 1 ,
 wherein at least one of the first optical filter and the second optical filter has an arrangement of a Mach-Zender filter.   
     
     
         3 . The wavelength monitor of  claim 1 ,
 wherein at least one of the first optical filter and the second optical filter has an arrangement of a ring filter.   
     
     
         4 . The wavelength monitor of  claim 1 ,
 wherein the transmittance of the first transmittance spectrum at respective grid wavelengths of the WDM system is substantially constant, wherein the transmittance of the second transmittance spectrum at respective grid wavelengths monotonically varies within the wavelength range of the WDM system.   
     
     
         5 . The wavelength monitor of  claim 4 ,
 wherein the first transmittance spectrum periodically varies by N cycles in the wavelength range of the WDM system and the second transmittance spectrum periodically varies by less than N cycles but greater than N−0.5 cycles in the wavelength range of the WDM system.   
     
     
         6 . The wavelength monitor of  claim 5 ,
 wherein the second transmittance spectrum periodically varies by less than N cycles but greater than N−0.1 cycles in the wavelength range of the WDM system.   
     
     
         7 . The wavelength monitor of  claim 1 ,
 wherein the wavelength monitor further includes a base PD, a first PD, and a second PD each of which are monolithically integrated with the first optical filter and the second optical filter;   wherein the first PD monitors light transmitted through the first optical filter, the second PD monitors light transmitter through the second optical filter, and the base PD directly monitors light generated in the tunable LD.   
     
     
         8 . A wavelength lockable LD, comprising:
 a tunable LD to emit light with an emission wavelength substantially coincident with a target grid wavelength of a WDM system;   a wavelength monitor including a first optical filter and a second optical filter, the first optical filter having a first transmittance spectrum periodically varying in a wavelength range of the WDM system, the second optical filter having a second transmittance spectrum periodically varying in the wavelength range, the first transmittance spectrum and the second transmittance spectrum at the target grid wavelength having a combination in respective transmittances specific to the target grid wavelength different from combinations in transmittances at other grid wavelengths;   a controller to tune the emission wavelength of the tunable LD such that the first optical filter and the second optical filter show a combination in respective transmittances equal to the combination at the grid wavelength.   
     
     
         9 . The wavelength lockable LD of  claim 8 ,
 wherein the first optical filter and the second optical filter has an arrangement selected from a group of a ring filter and a Mach-Zender filter.   
     
     
         10 . The wavelength lockable LD of  claim 8 ,
 wherein the first transmittances spectrum at respective grid wavelengths have substantially constant transmittance in the wavelength range, and   wherein the second transmittance spectrum at respective grid wavelengths have transmittances monotonically varying in the wavelength range.   
     
     
         11 . The wavelength lockable LD of  claim 10 ,
 wherein the first transmittance spectrum varies N cycles in the wavelength range, and the second transmittance spectrum varies less than N cycles but greater than N−0.5 cycles in the wavelength range.   
     
     
         12 . The wavelength lockable LD of  claim 11 ,
 wherein the second transmittance spectrum varies less than N cycles but greater than N−0.1 cycles in the wavelength range.   
     
     
         13 . The wavelength lockable LD of  claim 8 ,
 wherein the tunable LD has an arrangement selected from a group of a Fabry-Perot LD and a DFB LD, and   wherein the controller controls a temperature of the tunable LD to tune the emission wavelength thereof.   
     
     
         14 . The wavelength lockable LD of  claim 8 ,
 wherein the tunable LD includes a gain region, and two SG-GBR regions putting the gain region therebetween, and   wherein the controller controls biases each applied to the gain region and two SG-DBR regions to tune the emission wavelength of the tunable LD.   
     
     
         15 . The wavelength lockable LD of  claim 8 ,
 wherein the tunable LD includes a SG-DFB region and a CSG-DBR region, and   wherein the controller controls biases applied to the SG-DFB region and heater powers supplied to heaters monolithically integrated in the CSG-DBR region to tune the emission wavelength of the tunable LD.   
     
     
         16 . A method to tune an emission wavelength of a wavelength lockable LD to a target grid wavelength of a WDM system, the wavelength lockable LD including a tunable LD and a wavelength monitor monolithically integrated with the tunable LD, the wavelength monitor including a first optical filter having a first periodic transmittance spectrum and a second optical filter having a second periodic transmittance spectrum, the first periodic transmittance spectrum and the second periodic transmittance spectrum having a specific combination in respective transmittances at the target grid wavelength different from combinations in transmittances at other grid wavelengths, the method comprising steps of:
 guiding light generated by the tunable LD to the first optical filter and the second optical filter;   detecting a first output of the first optical filter, and a second output of the second optical filter;   tuning the emission wavelength of the tunable LD based on the first output so as to set the first output equal to a first preset transmittance for the first periodic transmittance spectrum; and   verifying the emission wavelength of the tunable LD by comparing the second output with a second preset transmittance of the second periodic transmittance spectrum,   wherein the first preset transmittance and the second preset transmittance constitute the specific combination at the target grid wavelength.   
     
     
         17 . The method of  claim 16 ,
 further including steps of, when the first output and the second output are offset from the specific combination, tuning the emission wavelength based on the second output and tuning again the emission wavelength based on the first output again after verifying the emission wavelength.   
     
     
         18 . The method of  claim 16 ,
 wherein said step of tuning the emission wavelength includes a step of varying a temperature of the tunable LD to tune the emission wavelength thereof when the tunable LD has an arrangement selected from a group of Fabry-Perot type and DFB type.   
     
     
         19 . The method of  claim 16 ,
 wherein said step of tuning the emission wavelength includes a step of varying biased supplied to the tunable LD to tune the emission wavelength thereof when the tunable LD includes a SG-DBR region.   
     
     
         20 . The method of  claim 16 ,
 wherein said step of tuning the emission wavelength includes a step of varying heater power supplied to heaters monolithically integrated in the tunable LD to tune the emission wavelength thereof when the tunable LD includes a CSG-DBR region.