Method and apparatus for an optical filter

• US 6,684,002 B2
• Filed: 08/31/2001
• Issued: 01/27/2004
• Est. Priority Date: 02/14/2001
• Status: Expired due to Term

• Current Assignee: II-VI Delaware, Inc. (Coherent Corp.)
• Original Assignee: Finisar Corporation (II-VI Incorporated)
• Inventors: Wang, Wei, Xie, Ping, Bettman, R. Brad, Wu, Simon Yuanxiang, Mao, Yalan, Maluf, Nadim
• Primary Examiner(s): SIKDER, MOHAMMAD YUNUS
• Application Number:
  US09/944,037
• Publication Number:
  US 20020154850A1
• Time in Patent Office: 879 Days
• Field of Search: 385/15, 385/3, 385/16, 385/20, 385/21, 385/24, 385/14, 385/31, 385/39, 385/42, 385/50, 359/113, 359/114
• US Class Current: 385/15
• CPC Class Codes: G02B 2006/12109 Filter
• CPC Class Codes: G02B 2006/12116 Polariser; Birefringent
• CPC Class Codes: G02B 2006/12164 Multiplexing; Demultiplexing
• CPC Class Codes: G02B 6/12007 forming wavelength selective elements, e.g. multiplexer, demultiplexer
• CPC Class Codes: G02B 6/272 comprising polarisation means for beam splitting and combining
• CPC Class Codes: G02B 6/2773 Polarisation splitting or combining
• CPC Class Codes: G02B 6/2861 using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering delay lines in general H03H, H01P9/00; optical computing devices G06E
• CPC Class Codes: G02B 6/29302 based on birefringence or polarisation, e.g. wavelength dependent birefringence, polarisation interferometers
• CPC Class Codes: G02B 6/2935 Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
• CPC Class Codes: G02B 6/29355 Cascade arrangement of interferometers
• CPC Class Codes: G02B 6/29386 Interleaving or deinterleaving, i.e. separating or mixing subsets of optical signals, e.g. combining even and odd channels into a single optical signal
• CPC Class Codes: G02B 6/29394 Compensating wavelength dispersion G02B6/29376 takes precedence; dispersion compensated optical transmission systems H04B10/2507

Abstract

An optical device that can be used in a range of telecommunications applications including optical multiplexers/demultiplexers and optical routers. The optical device splits and combines optical signals of frequency division multiplexed optical communication channels which are evenly spaced apart in frequency from one another. The optical device includes a first filter and a second filter. The first filter splits and combines odd and even channels depending on the propagation direction of the optical signal. The first filter exhibits complementary phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd and even set of channels and with integer multiples of a full wavelength for each center wavelength associated with a remaining one of the odd set and the even set. The second filter couples with the first filter to filter the odd and even sets of channels with phase retardations complementary to those experienced by the odd and even set of channels in the first filter. This complementary filtration has the effect of reducing dispersion in the device.

Claims

1. An optical device for splitting and combining optical signals, the optical device comprising:

a first filter for splitting and combining odd and even channels depending on a propagation direction, the first filter exhibiting complementary phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd set of channels and the even set of channels and corresponding with integer multiples of a full wavelength for each center wavelength associated with a remaining one of the odd set and the even set; and

a second filter coupled with the first filter to filter the odd and even sets of channels with phase retardations complementary to those experienced in the first filter.

2. The optical device of claim 1, wherein the first and second filters each comprise:

a pair of delay paths exhibiting an optical pathlength difference corresponding inversely with a spacing between adjacent odd channels and adjacent even channels, wherein the optical pathlength difference of the pair of delay paths of the first filter differ from the optical pathlength difference for the pair of delay paths of the second filter by an odd integer multiple of λ

/2 thereby to effect complementary polarization mode dispersions of the optical signal within the first filter together with the second filter.

3. The optical device of claim 1, wherein the first and second filters each comprise:

a pair of delay paths including a fast delay path and a slow delay path; and

couplers at opposing ends of the pair of delay paths to asymmetrically split and combine light between the pair of delay paths.

4. The optical device of claim 1, wherein each of the first and second filters includes:

a fundamental filter cell including a pair of optical paths for processing the optical communications, wherein the pair of optical paths exhibit an optical pathlength difference determinative of a free spectral range substantially corresponding with a frequency spacing between adjacent odd or adjacent even channels.

5. The optical device of claim 1, wherein the first and second filters each include:

a pair of filter cells serially coupled to one another with a fundamental one of the pair of filter cells exhibiting a fundamental free spectral range corresponding with a spacing between adjacent odd and even channels and a harmonic one of the pair of filter cells exhibiting a harmonic free spectral range corresponding to an integer fraction of the spacing between adjacent odd or even channels.

6. The optical device of claim 1, wherein the first and second filters each include:

two pairs of delay paths for optically processing the odd and even channels, each of the two pairs of delay paths exhibiting optical pathlength differences which correspond to a sum of a Fourier series including a fundamental frequency component corresponding with a spacing between adjacent odd or even channels; and

optical couplers interlaced with the two pairs of delay paths to asymmetrically split and combine channels across each of the two pairs of delay paths to effect an adjustment of stopbands between adjacent odd channels and adjacent even channels.

7. The optical device of claim 6, wherein the optical couplers include at least one of:

polarization couplers which split and combine optical signals depending on a polarization thereof and intensity couplers which split and combine optical signals depending on an intensity thereof.

8. The optical device of claim 6, wherein asymmetries of coupling in the first filter and the second filter correspond with one another.

9. The optical device of claim 1, wherein the first and second filters each comprise:

a birefringent crystal having an optical axis normal to a propagation path of the optical signal, the birefringent crystal splitting and combining odd and even channels depending on a propagation direction and an optical pathlength difference of extraordinary and ordinary ray paths of the first filter differing from corresponding extraordinary and ordinary ray paths of the second filter by an odd integer multiple of λ

/2.

10. An optical device for splitting and combining optical signals with frequency division multiplexed optical communication channels evenly spaced apart in frequency from one another;

and the optical device comprising;

a first filter for splitting and combining odd and even channels depending on a propagation direction and the first filter exhibiting complementary phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd set of channels and the even set of channels and corresponding with integer multiples of a full wavelength for each center wavelength associated with a remaining one of the odd set and the even set; and

at least a second filter coupled with the first filter to filter the odd set of channels with phase retardations complementary to those experienced by the odd set of channels in the first filter and to filter the even set of channels with phase retardations complementary to those experienced by the even set of channels in the first filter.

11. The optical device of claim 10, wherein the first and at least second filters each comprise:

at least one pair of delay paths exhibiting an optical pathlength difference corresponding inversely with a spacing between adjacent odd channels and adjacent even channels and the optical pathlength difference of the at least one pair of delay paths of the first filter differing from corresponding optical pathlength differences for the at least one pair of delay paths of the at least second filter by an odd integer multiple of λ

/2 thereby to effect complementary polarization mode dispersions of the optical signal within the first filter together with the at least second filter.

12. The optical device of claim 10, wherein the first and at least second filters each comprise:

at least one pair of delay paths including a fast delay path and a slow delay path; and

couplers at opposing ends of the at least one pair of delay paths to asymmetrically split and combine light between the at least one pair of delay paths, to effect a broadening of stopbands between adjacent odd channels and adjacent even channels.

13. The optical device of claim 10, wherein each of the first and at least second filters includes:

a fundamental filter cell including a pair of optical paths for processing the optical communications, and with the pair of optical paths exhibiting an optical pathlength difference determinative of a free spectral range substantially corresponding with a frequency spacing between adjacent odd or adjacent even channels.

14. The optical device of claim 10, wherein the first and at least second filters each include:

at least a pair of filter cells serially coupled to one another with a fundamental one of the pair of filter cells exhibiting a fundamental free spectral range corresponding with the spacing between adjacent odd and even channels and a harmonic one of the pair of filter cells exhibiting a harmonic free spectral range corresponding to an integer fraction of the spacing between adjacent odd or even channels.

15. The optical device of claim 10, wherein the first and at least second filters each include:

at least two pairs of delay paths for optically processing the odd and even channels, and each of the at least two pairs exhibiting optical pathlength differences which correspond to a sum of a Fourier series including a fundamental frequency component corresponding with the spacing between adjacent odd or even channels; and

optical couplers interlaced with the at least two pairs of delay paths to asymmetrically split and combine the communication channels across each of the at least two pairs of delay paths to effect a broadening of stopbands between adjacent odd channels and adjacent even channels.

16. The optical device of claim 15, wherein the optical couplers further comprise at least one of polarization couplers which split and combine optical signals depending on a polarization thereof and intensity couplers which split and combine optical signals depending on an intensity thereof.

17. The optical device of claim 15, wherein asymmetries of coupling in the first filter and the at least second filter correspond with one another.

18. The optical device of claim 10, wherein the first and at least second filters each comprise:

at least one birefringent crystal having an optical axis normal to a propagation path of the optical signal and the at least one birefringent crystal splitting and combining odd and even communication channels depending on a propagation direction and the optical pathlength difference of the extraordinary and ordinary ray paths of the first filter differing from corresponding “

e” and

“

o”

ray paths of the at least second filter by an odd integer multiple of λ

/2 thereby to effect complementary wavelength dispersions of the optical signal within the first filter together with the at least second filter.