IL104877A - Wavelength division multiplexing/demultiplexing system - Google Patents

Abstract

An optical signal transmission system for the transmission of optical signals of a plurality of channels (141-14n) of different wavelengths between (a) a plurality of optical transmission paths, one for each channel, and (b) a single optical transmission path (13) for all the channels, comprising: a holographic device (2) linking said plurality of optical transmission paths with said single optical transmission path; said holographic device being capable of trapping said optical signals within it by internal reflection, and having recorded thereon: a single hologram aligned with said single optical transmission path and including a plurality of holographic optical elements, one for each of said channels; and a plurality of holograms, each alinged with one of said plurality of optical transmission paths and having a holographic optical element for the channel of the respective path. 6 כ" ד באדר התשס" א - March 19, 2001

Description

104877/3 ran jrnnn ¾ w npten!? noura WAVELENGTH DIVISION MULTIPLEXING/ DEMULTIPLEXING SYSTEM YEDA RESEARCH & DEVELOPMENT CO. LTD. a"va nir Di ιρηο ντ Inventors: Yaakov Amitai Asher A. Friesem C: B8243 WAVELENGTH DIVISION MULTIPLEXING/DEMULTIPLEXING SYSTEM WAVELENGTH DIVISION MULTIPLEXING/DEMULTIPLEXING SYSTEM FIELD AND BACKGROUND OF THE INVENTION The present invention relates to optical signal transmission systems, and particularly to a novel wavelength division multiplexing/demultiplexing system.

Recently, there have been significant advances in optical fibers technology for telecommunication systems. One of the proposed methods to exploit more efficiently the high potential bandwidth of optical fibers is by wavelength division multiplexing (WDM) . With this technique, a large number of communication channels can be transmitted simultaneously over a single fiber. During the last decade, various systems for implementing WDM have been proposed, including systems based on birefringent materials, surface relief gratings, Mach-Zender interferometry , and waveguides. Unfortunately, these proposed systems generally suffer from low efficiencies or from a strict limitation on the number of channels .

Another proposed approach is to use a thick reflection hologram as described in N.Moslehi, P.Harvey, J.Ng and T.Jannson, Opt. Lett. 14,(1989) 1088. However, the necessity to use a conventional aspheric lens for collimating and/or focusing the light waves makes the system bulky and space consuming. Furthermore, a single holographic element is very sensitive to the signal's wavelength which usually depends strongly on temperature. - 2 - OBJECT AND BRIEF SUMMARY OF THE INVENTION An object of the present invention is to provide a novel optical signal transmission system which may be used for multiplexing and/or demultiplexing optical signals by the wavelength division multiplexing technique.

According to the present invention, there is provided an optical signal transmission system for the transmission of optical signals of a plurality of different channels of different wavelengths between (a) a plurality of optical transmission paths, one for each channel, and (b) a single optical transmission path for all the channels, comprising: a holographic device linking the plurality of optical transmission paths with the single optical transmission path; the holographic device being capable of trapping the optical signals within it by internal reflection, and having recorded thereon: a single hologram aligned with the single optical transmission path and including a plurality of holographic optical elements, one for each of the channels; and a plurality of holograms, each aligned with one of the plurality of optical transmission paths and having a holographic optical element for the channel of the respective path.

The system may be used for demultiplexing the optical signals transmitted from the single optical transmission path to the plurality of optical transmission paths. In such case, the single hologram is aligned with the single optical transmission path, and its plurality of holographic optical elements are effective to collimate the - 3 - light of the plurality of channels and to diffract the light in a different direction for each channel; in addition, each of the plurality of holograms of the holographic device is aligned with one of the plurality of optical transmission paths, and its holographic optical element is effective to focus the collimated light of the respective channel onto its respective optical transmission path.

The system may also be used for multiplexing the optical signals transmitted from the plurality of optical transmission paths to the single optical transmission path. In such case, each of the plurality of holograms of the holographic device is aligned with one of the plurality of optical transmission paths, and its holographic optical element is effective to collimate the light of the respective channel and to diffract it in a different direction for each channel; in addition, the single holograph of the holographic device is aligned with the single optical transmission path, and its plurality of holographic optical elements are effective to focus the collimated light of the respective channel onto , the single optical transmission path.

The invention may also be incorporated in a complete multiplexing/demultiplexing system, wherein the optical signals from a plurality of channels of different wavelengths are derived from a plurality of optical fibers, are multiplexed onto a single optical fiber, and are subsequently demultiplexed from the single optical fiber onto a plurality of optical fibers. - 4 - As will be described more particularly below, such a multiplexing and/or demultiplexing system can accommodate a large number of channels with small spectral separation, high efficiency, and neglible cross-talk between the channels. In addition, both the design and the recording procedure are fairly simple and do not have to resort to complicated and expensive equipment, such as computer-generated-holograms or aspherical lenses. Further, the recording technique is suitable for mass production, and the system can be very compact and easy to use.

Still further features and advantages of the invention will be apparent from the description below.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein : Fig. 1 is a diagram helpful in explaining the novel system of the present invention; , Fig. 2 is a side view diagrammatically illustrating a wavelength division demultiplexing system in accordance with the present invention; Fig. 3 is a top view diagrammatically illustrating the demultiplexing system of Fig. 2; Figs. 4, 5 and 6 are curves illustrating various relationships helpful in understanding the invention and its advantages, as will be described more particularly below; - 5 - Fig. 7 is a table also helpful in understanding the invention and its advantages, as to be described more particularly below; Fig. 8 diagrammatically illustrates a demultiplexer sytem according to Figs. 2 and 3 included in combination with a corresponding multiplexer system; and Figs. 9 and 10 are views corresponding to Figs. 2 and 3, respectively, but of the multiplexer system of Fig. 8.

DESCRIPTION OF A PREFERRED EMBODIMENT Basic Building Block (Fig. 1) The basic building block of the. novel system is illustrated in Fig. 1. This basic building block includes a holographic device 2 including a light transmissive substrate or plate 2a having an emulsion coating 2b thereon linking a source fiber 3 and a receiving fiber 4. The holographic device 2 has recorded on its emulsion coating two identical holographic optical elements (HOEs) H and H . The first HOE Hs collimates the light emerging from the source fiber 3 into a plane wave which is then trapped inside the plate by total internal reflection. The second HOE H focuses the collimated wave onto a receiving fiber 4. As shown by the publication Y.Amitai and J.W.Goodman, Appl . Opt. 30,(1991) 2376, which is hereby incorporated by references, such a building block can be recorded with predistorted wavefronts to achieve nearly diffraction limited imaging and high diffraction efficiencies, even in the presence of a recording-readout wavelength shift .

The Implementation of Figs . 2-7 Figs . 2 and 3 ilustrate the basic building block of Fig . 1 constructed to provide a wavelength division demultiplexing system 1 0 , including a holographic plate 1 2 linking a single source fiber 1 3 and a plurality of receiving fibers 1 ^ — ^η · The source fiber 1 3 contains n different communication channels, C\ . . . Cn, with the wavelengths λι . . . A„, respectively. The central hologram is composed of n different HOEs, Ή . . . Ή^, which collimate the corresponding incoming channels, and diffract them into different directions. Each channel C{ is then focused by its respective HOE, Ή^, onto its receiving fiber. It is evident that the propagation direction of the waves can be inverted to yield a system which multiplexes a number of channels from their separated source fibers onto one receiving fiber. Since the holographic plate can be located very close to the fibers, and the light waves are guided inside the holographic plate, the system can be very compact and easy to use. Furthermore, since the chromatic dispersion of Ή can be corrected for each channel by Ή[, the system is much less sensitive to source wavelength shifts.

In order to achieve high efficiency and negligible cross-talk between the chan- - 7- nels, each Ή must have high diffraction efficiency for its respective wavelength λ;, and very low efficiency for the other wavelengths A , j i. As it was shown before [7], each HOE can satisfy the Bragg condition for its appropriate wavelength, but to assure high diffraction efficiencies the relation [8] also must be fulfilled, where r?,- is the refraction-index modulation, D is the emulsion thickness, β0 and βίτης are the off-axis angles inside the emulsion of the reconstruc-Ψ tion and the image waves respectively, and m is an integer number. Hence, for a given D, βα, and (iimg , the necessary refraction-index modulation to achieve high diffraction efficiency is To assure that the output wave will be trapped inside the plate by total internal reflection, /3img must satisfy the relation 1 > sin /?,ms > i , (3) where v is the refraction index of the plate. Fig^ 4* shows the calculated wave¬ length sensitivity for a HOE recorded according to the following parameters: D = 15 μπι , dh = 6 mm , df = 24 mm , 1/ = 1.51 , 77,· = 0.017 (4) Xi = 633 nm , β0 = 0° , = 45° , -8- where is the diameter of each hologram, and df is the distance between the fibers and the hologram. It is apparent that for a small wavelength shift, up to ±5 nm, the efficiency is still above 90%, but for larger wavelength shifts it falls rapidly to zero. It has been shown before [9] that the efficiency is zero when Δλ;, the relative change in wavelength, is where the grating period of Ίί\ is Ai (6) 2ν άτι ψ- Inserting Eq. (6) into Eq. (5) yields where we assume that βα = 0 (i.e. the fibers are normal to the hologram plane).

Inserting Eq. * (4) into Eq. (7) yields Δλι = 38 nm, in accordance with Fig. 3. Equation (7) yields the desired channel spectral separation ΔΑ.;, which can be decreased by taking a thicker emulsion layer or by increasing j3img .

Another important parameter is the number of the channels that this WDM system can handle simultaneously. This number is actually the number of different HOEs which can be multiplexed together on the same substrate without reaching the refraction-index saturation of the recording material. Namely, the total sum of the desired refractive- index modulation for all the multiplexed channels must be less then the allowed maximum index modulation ητηαχ of the recording material. It has been shown before [10, 11] that for good recording materials like Dichromated Gelatin, when the relation n is fulfilled, a large number of holograms can be multiplexed together on the same substrate with high efficiencies, negligible absorption and with no index saturation. Inserting Eq. (8) into Eq. (2) yields the allowed maximum number of channels Nc , (9) where Figure 4(a) shows the spectral separation and the number of channels as a function of D for Xave = 633 nm (solid lines), and for Xa Ve = 1-4 μπι (dashed line), where max = 0.08 and img = 45°. The results reveal that a smaller spectral separation Δλί and a larger number of channels Ncftn can be achieved by increasing D. A much thicker emulsion must be taken for λαυε = 1-4 μτη in order to achieve the same performance as for Xa ve = 633 nm. Figure 4(b) shows the same calculations for fiimg — 60°. The improvement in the performances for both wavelengths is apparent. For example, a substrate with emulsion thickness of D = 40 μτη and with fiimg = 60° can accommodate 15 channels with a small spectral separation of Δλ,· ~ 7 nm. ■ - 16- Design Illustration and Experimental R The design procedure is illustrated experimentally with a two-channel WDM system, where the recording material is Dichromated Gelatin with emulsion thickness of D = 15 μπι. The system has the same parameters as in Eq. (4) where λι = 633 nm and A2 = 595 nm. The four HOEs: K{, H2a (which were multiplexed together), 7ί[, a.nd were recorded according to the recursive procedure [7], where the recording wavelength was Arec = 458 nm. It is apparent that Ή\ is identical to Ή , only the reconstruction and the image waves are exchanged. Therefore, we need the same recording procedure for both holograms. Since the recording wavelength is different from the readout wavelength, the holographic, elements must be recorded with pre-distorted wavefronts, in order to assure high diffraction efficiencies and low aberrations. The pre-distorted wavefronts are derived from interim holograms whose readout geometries differ from those used during recording. Specifically, the aberrated object and reference waves are derived from intermediate holograms, Ή0^ and i[e , respectively (the superscripts obj and ref also denote all the parameters that are related to Ή0^ and respectively).

We assume (to be proved below) that each multiplexed HOE, Ή (i = 1, 2), is very efficient for its respective wavelength Α,· and actually transparent for the other wavelength A , j i. Hence, since each HOE is acting only on a single wavelength, we may use the design procedure which is described in details in reference [7].

According to this design, the relations that describe the relevant parameters of the interim holograms to yield high efficiencies and diffraction limited imaging are sin °bj = -(a -f Δ,,^^,. ) sin ?,m3 , re R (10) l l i_ _ l \ + il HR°objy + (R°cbj (Roref (Rrcef)3 J_ ( 1 1 1 1 \ R5c + ^ (R°0bj)5 + (R°cbjy. {R?ff {R?ff J Robj = ^re/ = ∞ where c, o, and r are the indices of the reconstruction, object and reference wave, respectively, Rq (q — c, o, r) is the distance between the respective point source and the center of the hologram, β9 is the respective off-axis angle outside the holographic plate, and μ, a, 6, /?,mfl , and Δν,μ,βί are defined as — — ^ A„ ' sin m,, Inserting the values from Eq. (4) into Eq. (10), and setting μ = 633/458 = 1.38, yields the following parameters for 7ί° 3 and R0°bj = -33.3 mm, β?> = -69.75°, R°cbj = 27.2 mm, (12) = -188.3 mm, = -9.83°, Rcref = 23.7 mm.

We repeated the same procedure for recording Ή and Ή2Γ with the interim holograms Ή^' and Ή.™ ■> where we set now μ = 595/458 = 1.30. The relevant parameters for Ή0 and Ή ^ are now H.°6 = -32.4 mm, #6i = -73.79°, R°cbj = 27 A mm, (13) Rraef = -224.4 mm, β 1 = -8.28°, = 23.7 mm.

Table 1 illustrates the efficiencies of the .various HOEs for λι and - It is apparent : that each HOE.is efficient for. its . respective wavelength and essentiall transparent to the second wavelength. The total diffraction efficiency of both chan¬ nels is more than 50%, and the cross-talk between the channels, taking into account also the de-focusing and the lateral shift, is practically zero. We also measured the spot sizes of the two waves, λι and A2, which were focused by Ή and ΉΓ respec¬ tively. Both spot sizes were measured to be ~ 7 μm, which is nearly a diffraction limited performance. · Although the experimental results were demonstrated for a system with two channels and moderate spectral separation, the procedure can readily be extended, according to the theoretical results in the previous section, to fabricate WDM sys¬ tems with large numbers of channels and much smaller spectral separation. •Γ3- The Multiplexer/Demultiplexer System of Figs. 8-10 Fig. 8 diagrammatically illustrates a demultiplexer 10 including a source fiber 13 and a plurality of receiving fibers 14- — 14 as described above, 3 1 n incorporated into a complete multiplexer/demultiplexer system. Such a system includes a multiplexer 20 designated 20 which receives the optical signals from a plurality of channels of different wavelengths, as supplied by a plurality of source fibers 21^--24n, and multiplexes the optical signals onto the single fiber 13 serving as an intermediate transmission fiber. Demultiplexer 10 demultiplexes all the optical signals on the intermediate fiber 13 back to their respective optical fibers.

Multiplexer 20 in Fig. 8. is more particularly illustrated in Figs. 9 and 10, wherein it will be seen that it corresponds to the illustrations in Figs. 2 and 3 but with an inverted propagation direction of the waves to produce a multiplexing operation of a plurality of separate channels to a single channel, rather than a demultiplexing operation of a single channel to a plurality of separate channels .

While the invention has been described with respect to one preferred embodiment, it will be appreciated that many variations and other applications of the invention may be made.

Claims (9)

WHAT IS CLAIMED IS:

1. An optical signal transmission system for the transmission of optical signals of a plurality of channels of different wavelengths between (a) a plurality of optical transmission paths, one for each channel, and (b) a single optical transmission path for all the channels, comprising: a holographic device linking said plurality of optical transmission paths with said single optical transmission path; said holographic device being capable of trapping said optical signals within it by internal reflection, and having recorded thereon: a single hologram aligned with said single optical transmission path and including a plurality of holographic optical elements, one for each of said channels; and a plurality of holograms, each aligned with one of said plurality of optical transmission paths and having a holographic optical element for the channel of the respective path.

2. The system according to Claim 1, wherein said system is a demultiplexing system for demultiplexing the optical signals transmitted from the single optical transmission path to the plurality of optical transmission paths; said single hologram of the holographic device being aligned with said single optical transmission path, and its plurality of holographic optical elements being effective to collimate the light of the plurality of channels and to diffract said light in a different direction for each channel; each of said plurality of holograms of the holographic device being aligned with one of said plurality of optical transmission paths, and its holographic optical element being effective to focus the collimated light of the respective channel onto its respective optical transmission path.

3. The system according to Claim 1 , wherein said system is a multiplexing system, multiplexing the optical signals transmitted from the plurality of optical transmission paths to the single optical transmission path; each of said plurality of holograms of the holographic device being aligned with one of said plurality of optical transmission paths, and its holographic optical element being effective to collimate the light of the respective channel and to diffract it in a different direction for each channel; said single holograph of the holographic device being aligned with said single optical transmission path, and its plurality of holographic optical elements being effective to focus the collimated light of the respective channel onto the single optical transmission path. .

4. An optical signal transmission system comprising: a multiplexing system according to Claim 3 for multiplexing the optical signals transmitted through a plurality of optical transmission paths to the single optical transmission path; - lb- 104877/2 and a demultiplexing system according to Claim 2, following said multiplexing system, for demultiplexing the optical signals transmitted through the single optical transmission path to the plurality of optical transmission paths .

5. The system according to any one of Claims 1-4, wherein each of said optical transmission paths is defined by an optical fiber.

6. The system according to any one of Claims 1-5, wherein each of said holographic optical elements has high diffraction efficiency for the wavelength of its respective channel and low diffraction efficiency for all other wavelengths .

7. The system according to any one of Claims 1-6, wherein said holographic device includes a light transparent plate and an emulsion coating thereon on which said holograms are recorded.

8. ■ The system according to any of the preceding claims and substantially as shown and described hereinabove.

9. The system according to any of the preceding claims and substantially as shown and described in any of the drawings . . , voca es an atent Attorneys, P.O. Box 2273, Rehovot 76122. C: B8243