Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/29—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the position or the direction of light beams, i.e. deflection
  • G02F1/31—Digital deflection, i.e. optical switching
  • G02F1/313—Digital deflection, i.e. optical switching in an optical waveguide structure
  • G02F1/3132—Digital deflection, i.e. optical switching in an optical waveguide structure of directional coupler type

Definitions

• This invention relates to optical devices, and in particular to directional couplers used in Mach-Zehnder Interferometer (MZI) devices.
• MZI Mach-Zehnder Interferometer
• Integrated optical devices have many uses in the fields of optical communication, optical sensing and optical processing. These are optical components formed by depositing or fabricating waveguide structures on a planar substrate.
• This technique has many established advantage's over the use of discrete optical components based on fibre or bulk optical technology. Many separate functional components can be fabricated on a single substrate, saving manufacturing costs and allowing the temperature and other environmental conditions to be equalised across the group of components.
• the waveguides on an optical integrated circuit can often follow curves which are far too tight for optical fibres.
• the waveguide material can be chemically altered in various ways, or even substituted by a different material such as a polymer, without having to allow for the mechanical stress of the fibre drawing process or the need to maintain the physical strength or integrity of the waveguide.
• FIG. 1 of the accompanying drawings schematically shows a typical structure of an optical integrated circuit.
• a silicon substrate 10 has a thermally grown silica (SiO 2 ) buffer layer 15.
• a waveguide core 20 is fabricated by depositing a layer of silica, doped for higher refractive index, for example by flame hydrolysis deposition followed by a consolidation step, and then etching away all but the required waveguide core.
• a cladding material 25 such as silica but doped so as to be of lower refractive index than the waveguide core is then deposited over the waveguide core structure.
• a heating element 35 over the core is also illustrated.
• FIGS. 2a to 2b of the accompanying drawings schematically illustrate typical optical components formed using the above techniques.
• the components comprise input waveguides 40, a first optical coupler 45 functioning as a splitter, a pair of waveguide arms 50 and a second optical coupler 55, functioning as a combiner with these parts in combination forming a Mach-Zehnder interferometer (MZI) arrangement.
• MZI Mach-Zehnder interferometer
• an extra phase-shift of ⁇ can be introduced by making one of the waveguide's arms longer. This makes it possible in the unpowered (off-) state to switch input one to output one.
• the first option is to use symmetrical waveguides (both waveguides having the same width and shape) with a coupling length (L) of aL ⁇ where Lc is the coupling length for full coupling of a signal from one waveguide to the other.
• Another option is to use asymmetrical waveguides (both waveguides have different shape and/or widths), with coupling lengths of Lc.
• the advantage of the second option is that the structure is not so dependable on wavelength variation, because the variation in the coupling length is more tolerable.
• Figure 3 of the accompanying drawings shows the configuration of a directional coupler. This comprises two waveguides with curved input/output sections 70, 72 and a coupling region there between of length L. In the coupling region the two waveguides are separated by a distance 'S'.
• Figures 4 to 7 show various possible different types of coupler which can be achieved by changing the waveguide cores in the coupling region.
• Figure 4 shows a symmetric directional coupler in which the two arms of the coupler have the same cross-sectional geometry and refractive index profile;
• Figure 5 shows an asymmetric coupler with straight waveguides, but of different widths, Dl and D2;
• Figure 6 shows an asymmetric coupler with one tapered and one straight waveguide;
• Figure 7 shows an asymmetric coupler with two tapered waveguides.
• Symmetric couplers have very good coupling ratios, ranging from 0 to 1. However, they have a very wavelength dependent response (i.e. power output varies with wavelength of input light). In many applications, directional couplers with a substantially wavelength independent response or wavelength flattening properties(i.e. compensation for wavelength dependence), are desirable. For a symmetric directional coupler, the coupling coefficient only approximates to being wavelength independent in the wavelength range of from 1500 to 1600nm, for a difference between the refractive index of the waveguide core to that of its surrounding cladding, of 0.25%.
• the required waveguide dimensions and separation fall within possible fabrication tolerances. For example 5.8 ⁇ m for the waveguide width and 6.5 ⁇ m for the separation, 'S'.
• the required separation for example, becomes about 0.0 l ⁇ m and the waveguide widths about 5 ⁇ m.
• Such dimensions often do not meet bulk processing tolerances, and instead require dedicated and expensive fabrication processes to be used.
• the coupling coefficient is very high, resulting in a very short coupling length ( «350 ⁇ m).
• Asymmetric couplers of the type shown in Figure 7 are wavelength flattening because the phase difference between the odd and even mode accumulated over the first half of the coupler length is reversed over the second half.
• One disadvantage of these couplers is that the coupling ratio of these types of couplers is not as good as that of a symmetric coupler.
• Asymmetric couplers of the type shown in Figure 6 with one straight and one tapered waveguides have the worst properties.
• Table 1 shows an overview of the minimum and maximum coupling ratios of different kinds of couplers.
• the most important parameter of Table 1 is the minimum coupling ratio, because this determines the maximum extinction ratio. It is desirable for the maximum extinction ratio to be as large as possible.
• a non-zero coupling ratio means a lower extinction ratio. It can be seen from Table 1 that the configuration with two tapered waveguides is not suitable for obtaining high extinction ratio, although it does show a good wavelength response.
• the optical device of the present invention has been found to have improved characteristics.
• the present invention provides an optical device, such as a Mach-Zehnder Interferometer (MZI), comprising a substrate having two input waveguides connected to a first optical coupler, two output waveguides connected to a second optical coupler, and a pair of waveguide arms connected between the two couplers, wherein the first optical coupler comprises two waveguide portions, a first one of which has a greater relative width than the second, and wherein the second optical coupler comprises two waveguide portions, the first one of which has a smaller relative width than the second, the broader waveguide portion of the first coupler being connected by a first one of the waveguide arms to the narrower waveguide portion of the second coupler, and the narrower waveguide portion of the first coupler being connected by the second waveguide arm to the broader waveguide portion of the second coupler.
• MZI Mach-Zehnder Interferometer
• Figure 1 schematically illustrates a typical structure of an optical integrated circuit
• Figures 2a and 2b schematically illustrate typical Mach-Zehnder
• Figure 4 shows the configuration of a symmetrical coupler
• Figure 5 shows the configuration of an asymmetric coupler with straight waveguides
• Figure 6 shows the configuration of an asymmetric coupler with one tapered and one straight waveguide
• Figure 7 shows the configuration of an asymmetric coupler with two tapered waveguides
• Figure 8 shows an optical device according to an embodiment
• FIG. 9 represents schematically a possible configuration of a TOS in one embodiment of the invention.
• Figure 10 shows a cross-section through the arms of an MZI
• Figure 11 shows the configuration of a combination of 2x2 TOS switches according to another embodiment
• Figure 12 shows the layout of the waveguides in the switch of Figure 11;
• Figure 13 shows the possible layout of the waveguides in a single stage 2x2 TOS.
• Figure 14 shows the function of a low cross-talk switch.
• the optical device of an embodiment provides a planar substrate (not shown) on which there are two input waveguides 100, 110, connected to a first optical coupler 120, two output waveguides 130, 140, connected to a second optical coupler 150 and a pair of waveguides arms 160, 170 connected between the two couplers 120, 150.
• Each of the first and second couplers comprise a pair of waveguide portions 122, 124, 152, 154, each one of a pair having a relative different width to the other of the pair.
• the waveguide portions are arranged such that the first- coupler waveguide portion of greater relative width is connected to the second-coupler waveguide portion of smaller relative width; and the first-coupler waveguide portion of the smaller relative width is connected to the wider of the two second-coupler waveguide portions.
• the second coupler compensates for the phase-difference changes introduced by the first coupler, and has a compensating wavelength response to that of the first coupler, to produce a compensated wavelength-flattened response.
• the optical device functions as a switch, and the two couplers of the device each comprise two straight waveguides of different widths, the couplers being inverted with respect to one another.
• the system works through using a combination of the properties of Table 1.
• the devices of the preferred embodiments have the excellent wavelength flattening response for splitting of a straight asymmetric directional coupler, combined with the reversed accumulated phase-difference properties of a tapered directional coupler (Figure 7).
• the MZI's of figures 2a and 2b can each employ the directional coupler described above as the coupler (55) and/or splitter (45).
• a low power Thermo-Optic Switch can be made by introducing an extra ⁇ /2 phase shift (at a wavelength in the middle of the desired range).
• the bar state of the switch is defined as when a signal input on input 1 is output on output 1, with a signal input on input 2 being output on output 2.
• the cross state, or x-state is defined as a signal input on input 1 being output on output 2, with signals input on input 2 being output on output 1. It has been found that the bar-state and x-state of this TOS have a WDL
• the compensation configuration has two main advantages, which lead to its good x-state extinction ratio. Firstly the configuration compensates in the second splitter for any deviation from a 50% split by the first splitter. Secondly the phase- wavelength dependency of the heater can be compensated.
• VOA Variable Optical Attenuator
• Figures 13 show typical dimensions of a single stage MZI with asymmetrical, inverted directional couplers with input/output bends, showing the taper sections used to connect straight waveguide portions of greater width to curved portions of smaller width. These taper sections are required so that the width of the waveguides changes slowly enough that there is insignificant coupling of a transmitted signal with higher order modes. This is known as tapering adiabatically.
• a plurality of the above described TOS's can be used to make a 2x2 TOS switch as shown schematically in Figure 11.
• a crossing is needed. This is preferably at 90°, for optimal functionality and minimal cross talk, but this requires a larger overall structure of the switch.
• the crossing angle ⁇ can be reduced to as little as 20°, although ⁇ >30° is preferred.
• the crossing angle is about 20°
• the offset at the bends is about 650 ⁇ m
• the bend length is about 6500 ⁇ m with a cross-talk less than -45dB, a bend radius of 16.4 ⁇ m and an insertion loss of 0.14dB.
• Another example has a bend radius of 20 ⁇ m, and an insertion loss of 0.07dB.
• the preferred widths and separation of the two couplers making up a TOS switch are as follows:
• this 2x2 TOS switch has eight heaters in total, and uses three heaters to switch each time. In the no power state the switch works in the broadcast mode. To make a blocking switch the power consumption increases by about double. The function of this 2x2 switch is explained in Table 2 below.
• this configuration works as follows.
• the first single stage TOS gives an extinction ratio of 20 dB.
• the main power of the signal will go to the stage of output 1 , and the small unwanted signal propagates to the second stage TOS for output 2.
• the unwanted signal is further reduced by transferring this signal to the dummy output. This will give an extra 20 dB extinction ratio. So the total extinction ratio between the two outputs will become 40 dB.
• the present invention provides an optical device which provides surprisingly good results combining a wavelength flattening response with a high extinction ratio.
• the device has many applications only some of which are mentioned here, but it finds particular use in TOS switches and VOAs.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Integrated Circuits (AREA)

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Cited By (11)

Citations (3)

• 2001
  • 2001-02-06 GB GB0102957A patent/GB0102957D0/en not_active Ceased
• 2002
  • 2002-02-05 EP EP02710182A patent/EP1259855A1/de not_active Withdrawn
  • 2002-02-05 WO PCT/GB2002/000494 patent/WO2002063389A1/en not_active Application Discontinuation

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