Birefringent optical wavelength multiplexer/demultiplexer

• US 4,566,761 A
• Filed: 09/13/1984
• Issued: 01/28/1986
• Est. Priority Date: 09/13/1984
• Status: Expired due to Fees

• Current Assignee: GTE Laboratories Incorporated (Lumen Technologies, Inc.)
• Original Assignee: GTE Laboratories Incorporated (Lumen Technologies, Inc.)
• Inventors: Melman, Paul, Carlsen, W. John
• Primary Examiner(s): ARNOLD, BRUCE
• Application Number:
  US06/650,012
• Time in Patent Office: 502 Days
• Field of Search: 350/401, 350/403, 370/2, 455/616
• US Class Current: 359/489.07
• CPC Class Codes: G02B 27/283 used for beam splitting or combining
• CPC Class Codes: G02B 27/288 Filters employing polarising elements, e.g. Lyot or Solc filters G02B5/3016 takes precedence
• CPC Class Codes: G02B 6/105 having optical polarisation effects
• CPC Class Codes: G02B 6/272 comprising polarisation means for beam splitting and combining
• CPC Class Codes: G02B 6/2766 Manipulating the plane of polarisation from one input polarisation to another output polarisation, e.g. polarisation rotators, linear to circular polarisation converters
• CPC Class Codes: G02B 6/2773 Polarisation splitting or combining
• CPC Class Codes: G02B 6/2817 using reflective elements to split or combine optical signals
• CPC Class Codes: G02B 6/29302 based on birefringence or polarisation, e.g. wavelength dependent birefringence, polarisation interferometers

Abstract

An optical multiplexer/demultiplexer provides for arbitrarily closely spaced wavelength channels, the spacing and locations thereof being determined by the thickness of birefringent crystals such as, for example, quartz. Such a device can also be used for duplexing bidirectional signals. Methods for mechanically tuning the devices to given laser wavelengths are described, as well as simplified constructions which can be used with polarized light sources such as lasers.

Claims

1. A birefringent optical wavelength multiplexer/demultiplexer comprising

first polarization beam splitting means for splitting an applied collimated beam of light into two linearly polarized beams whose directions of propagation are perpendicular to one another;

first reflecting means for reflecting one of said linearly polarized beams parallel to the other polarized beam;

a birefringent element having a pair of opposed parallel surfaces, with an optical axis oriented nominally parallel to said surfaces, said element being oriented with said parallel surfaces perpendicular to the direction of propagation of said linearly polarized beams, said optical axis being oriented half way between the two polarized beams incident upon said element, 45°

from the polarization axis of each said beam, wherein

one electric field component of each of said polarized beams which is parallel to an "extraordinary" axis of said element propagates at a different rate of speed, dependent upon the material of said element, than the other electric field component which is parallel to an "ordinary" axis of said element;

second reflecting means for reflecting the other of said linearly polarized beams, following propagation of said other beam through said element, in a direction perpendicular to that of said one of said polarized beams following propagation of said one polarized beam through said element; and

second polarization beam splitting means oriented to receive both said reflected other beam and the propagated one beam, wherein

(1) when the propagated other beam and the propagated one beam have polarization states which are linearly polarized parallel to said respective applied polarized beams, said reflected other beam is transmitted by said second polarization beam splitting means, and said propagated one beam is reflected by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam are combined and transmitted outwardly from a first port; and

(2) when the propagated other beam and the propagated one beam have polarization states which are linearly polarized perpendicular to said respective applied polarized beams, said reflected other beam is reflected by said second polarization beam splitting means, and said propagated one beam is transmitted by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam are combined and transmitted outwardly from a second port.

2. A birefringent optical wavelength multiplexer/demultiplexer comprising

first polarization beam splitting means for splitting an applied collimated beam of light into two linearly polarized beams whose directions of propagation are at an angle to one another;

first reflecting means for reflecting one of said linearly polarized beams parallel to the other polarized beam;

a birefringent element having a pair of opposed parallel surfaces, with an optical axis oriented nominally parallel to said surfaces, said element being oriented with said parallel surfaces perpendicular to the direction of propagation of said linearly polarized beams, said optical axis being oriented half way between the two polarized beams incident upon said element, equiangular from the polarization axis of each said beam, wherein

one electric field component of each of said polarized beams which is parallel to an "extraordinary" axis of said element propagates at a different rate of speed, dependent upon the material of said element, than the other electric field component which is parallel to an "ordinary" axis of said element;

second reflecting means for reflecting the other of said polarized beams, following propagation of said other beam through said element, in a direction perpendicular to that of said one of said polarized beams following propagation of said one polarized beam through said element; and

second polarization beam splitting means oriented to receive both said reflected other beam and the propagated one beam, wherein

(1) when the propagated other beam and the propagated one beam have polarization states which are linearly polarized parallel to said respective applied polarized beams, said reflected other beam is transmitted by said second polarization beam splitting means, and said propagated one beam is reflected by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam are combined and transmitted outwardly from a first port; and

(2) when the propagated other beam and the propagated one beam have polarization states which are linearly polarized perpendicular to said respective applied polarized beams, said reflected other beam is reflected by said second polarization beam splitting means, and said propagated one beam is transmitted by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam are combined and transmitted outwardly from a second port.

3. A birefringent optical wavelength multiplexer/demultiplexer comprising

first polarization beam splitting means for splitting a first applied collimated beam of light into two linearly polarized beams whose directions of propagation are perpendicular to one another, and for splitting a second applied collimated beam of light into two linearly polarized beams whose directions of propagation are perpendicular, one polarized beam from said first collimated beam being directed along a first path common with one polarized beam of said second collimated beam, and the other polarized beam from said first collimated beam being directed along a second path common with the other polarized beam of said second collimated beam;

first reflecting means for reflecting said one polarized beams to a third path parallel to said second common path;

a birefringent element having a pair of opposed parallel surfaces, with an optical axis oriented nominally parallel to said surfaces, said element being oriented with said parallel surfaces perpendicular to said third path and said second common path, said optical axis being oriented half way between the one polarized beams of both collimated beams and the other polarized beams of both collimated beams incident upon said element, 45°

from the ones and the others beams, wherein

those components of said polarized beams which are parallel to an "extraordinary" axis of said element propagate at a different rate of speed, dependent upon the material of said element, than the other polarized components which are parallel to an "ordinary" axis of said element;

second reflecting means for reflecting said other polarized beams that have traversed said second common path to a direction perpendicular to said third path; and

second polarization beam splitting means oriented to receive the one polarized beams from both collimated beams that have traversed said third path, and said other polarized beams from both collimated beams that have traversed said second common path, wherein

(1) when both polarized beams from said first collimated beam combine to yield a polarization state which is linearly polarized parallel to said first applied collimated beam, the reflected other beam of said first collimated beam is transmitted by said second polarization beam splitting means, and said propagated one beam of said first applied collimated beam is reflected by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam of said first collimated beam are combined and transmitted outwardly from a first port;

(2) when both polarized beams from said second collimated beam combine to yield a polarization state which is linearly polarized perpendicular to said second applied collimated beam, the reflected other beam of said second collimated beam is transmitted by said second polarization beam splitting means, and said propagated one beam of said second collimated beam is reflected by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam of said second collimated beam are combined and transmitted outwardly from said first port;

(3) when both polarized beams from said first collimated beam combine to yield a polarization state which is linearly polarized perpendicular to said first applied collimated beam, the reflected other beam of said first collimated beam is reflected by said second polarization beam splitting means, and said propagated one beam of said first applied collimated beam is transmitted by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam of said first collimated beam are combined and transmitted outwardly from a second port; and

(4) when both polarized beams from said second collimated beam combine to yield a polarization state which is linearly polarized parallel to said second applied collimated beam, the reflected other beam of said second collimated beam is reflected by said second polarization beam splitting means, and said propagated one beam of said second collimated beam is transmitted by said second polarization beam splitting means, whereby said reflected other beam and said propagated one beam of said second collimated beam are combined and transmitted outwardly from said second port.

4. A comb filter unit comprising

a birefringent element having parallel opposite sides,

a first polarization beam splitter for splitting a collimated beam of light into two linearly polarized beams whose directions of propagation are at an angle to one another,

a first reflecting surface for reflecting one of said polarized beams along a path parallel to the other of said polarized beams,

both said other polarized beam and said reflected beam being incident upon one of said parallel sides, for transmission through said element,

a second reflecting surface for reflecting the other of said polarized beams following its transmission through the other of said parallel sides, and

a second polarization beam splitter for combining said reflected other beam and said reflected one beam and transmitting the combined light through one of two ports.

5. A mechanically tunable optical wavelength multiplexer including a comb filter unit comprising

a birefringent element having parallel opposite sides,

a first polarization beam splitter for splitting a collimated beam of light at a wavelength λ

into two linearly polarized beams whose directions of propagation are at an angle to one another,

a first reflecting surface for reflecting one of said polarized beams along a path parallel to the other of said polarized beams,

both said other polarized beam and said reflected beam being incident upon one of said parallel sides of said element for transmission therethrough,

a second reflecting surface for reflecting the other of said polarized beams following its transmission through the other of said parallel sides, and

a second polarization beam splitter for combining said reflected other beam and said reflected one beam and transmitting the combined light through one of two ports,

said birefringent element including a pair of wedges of birefringent material, arranged such that moving one relative to the other changes the effective thickness of the material through which the light propagates, and thus varies the overall retardation

whereby, a continuous change in both said wavelength λ and

in channel spacing Δ

λ

is provided, so that in a 2-channel system, said unit can be tuned precisely to the wavelengths of two signals to be multiplexed.

6. A mechanically tunable optical wavelength multiplexer comprising

a birefringent element having parallel opposite sides,

a first polarization beam splitter for splitting a collimated beam of light into two linearly polarized beams whose directions of propagation are at an angle to one another,

a first reflecting surface for reflecting one of said polarized beams along a path parallel to the other of said polarized beams,

both said other polarized beam and said reflected beam being incident upon one of said parallel sides of said element for transmission therethrough,

a quarter-wave plate oriented, adjacent to the other side of said element, at 45°

to an optical axis of said element, and

a rotatable half-wave plate;

a second reflecting surface for reflecting the other of said polarized beams, following its transmission through the other of said parallel sides, said quarter-wave plate, and said half-wave plate, and

a second polarization splitter for combining said reflected other beam and said reflected one beam, following their individual passage through said half-wave plate and transmitting the combined light through one of two ports,

whereby rotation of said half-wave plate tunes the entire wavelength passband over a range of approximately one full period.

7. A four channel optical wavelength multiplexer comprising

a birefringent element having opposed parallel sides;

a first polarization beamsplitter adapted to pass optical signals at a wavelength λ

₂ and to reflect optical signals at wavelength λ

₁, so that both signals at wavelengths λ

₂ and λ

₁ are directed along a first common path incident upon one of said parallel sides of said element;

a second polarization beamsplitter adapted to pass optical signals at a wavelength λ

₃ and to reflect optical signals at wavelength λ

₄, so that both signals at wavelengths λ

₃ and λ

₄ are directed along a second common path, parallel to said first common path, incident upon said one parallel side of said element;

a first reflecting surface for reflecting signals at wavelengths λ

₂ and λ

₁, that have traversed said first common path and said element, at an angle perpendicular to said first common path; and

a third polarization beam splitter for passing said signals at wavelengths λ

₁ and λ

₂, and for reflecting signals at wavelengths λ

₃ and λ

₄ that have traversed said second common path and said element, so that all signals λ

₁, λ

₂, λ

₃, and λ

₄ are outputted along a third common path.

8. A birefringent multiplexer/demultiplexer comprising a tree structure of comb filter units A, B, and C, wherein

two, A and B, of said comb filters have a basic channel spacing of 2 Δ

λ

, said comb filter A receives signals at wavelengths λ

₁ and λ

₃ along separate channels thereof and combines onto a first output path, said comb filter B receives signals at wavelength λ

₂ and λ

₄, along separate channels of said filter B, having the same channel spacing as said comb filter A but tuned a distance away therefrom, and combines said signals λ

₂ and λ

₄ onto a second output path;

said first and said second output paths being coupled to two separated ports of the third comb filter unit C which has a channel spacing equal to Δ

λ

;

whereby signals at λ

₁ and λ

₃ travel along the first path, and signals at λ

₂ and λ

₄ travel along the second path, and all four signals λ

₁, λ

₂, λ

₃ and λ

₄ are combined onto a third output path.