ES2697948A1 - TINTAL OPTICAL DELAY LINE AND METHOD FOR TUNING DELAY (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Tunable optical delay line and method for delay tuning. A tunable optical delay line (1) is described in this document while detailing a method of operation thereof that allows to implement linearly variable delays that extend to more than one optical channel. The invention described herein makes it possible to implement an arbitrary complex group delay with respect to optical frequency profiles over wide-band optical spectral regions spanning several hundred GHz. The invention is based on the implementation of a broadband tunable optical dispersive delay line by separate interpolation of the desired local group delay characteristic in each optical channel. The local group delay characteristic is implemented by a tuning technique of corresponding narrow-band separated channels where each optical channel is tuned separately in the delay. (Machine-translation by Google Translate, not legally binding)

Description

TINTAL OPTICAL DELAY LINE AND METHOD FOR TUNING

DELAY

D E S C R I P C I O N

OBJECT OF THE INVENTION

The object of the invention is part of the technical field of physics, more specifically in the technical field of optics.

More specifically, the invention relates to an optical delay line that can be tunable and can be integrated into an optical chip and configured to execute delays as a function of the frequency in wide-band optical spectral regions associated with optical channels, and its method of tuning the delay.

BACKGROUND OF THE INVENTION

Optical delay lines (ODLs) are physical elements linked to an optical fiber to generate a delay in an optical signal transmitted through the optical fiber.

Usually, this optical signal comprises an optical carrier signal with a carrier frequency that is modulated in radiofrequency (RF) by a subcarrier signal containing different modulation frequencies.

These optical delay lines are critical components for the implementation of many microwave photonics (MWP) functionalities, which are necessary in a wide variety of fields, including information and communication, biomedical, security and aerospace industries. Within these fields, the use of optical delay lines is common in reconfigurable wireless networks of optical band, reconfigurable photonic microwave filters, photonic generators of arbitrary radiofrequency waves and optoelectronic oscillators.

Currently, the optical delay lines are based on a device comprising independent ring resonators. Despite this, these devices have insufficient bandwidth for the typical values of the delays required in most photonic microwave applications. This limitation can be overcome within the Typical bandwidth of a single optical channel (frequency range of ± 50 GHz around the frequency of the optical carrier) by a technique known as separate carrier tuning (SCT).

An example of this technology is described by Z. Yu, X. Jin, J. Chen, G. Wang and DR Selviah, "Microring-based tunable optical delay lines for optical time-division multiplexers," 2015 11th Conference on Lasers and Electro -Optics Pacific Rim (CLEO-PR), Busan, 2015, pp. 1-2. doi: 10.1109 / CLEOPR.2015.7376463. In this document a tunable optical delay line based on resonator rings is detailed to be used in OTDM (optical time division multiplexer) systems. It is a method that seeks to increase the bandwidth of a resonant delay by means of a controlled detuning of the resonant retarders implemented by means of ring structures.

Despite this, this method is restricted to an optical channel and not to multiple channels. In addition, the delay affects the entire spectrum of the channel and is a constant value. That is, it can not implement linearly variable delays that extend to more than one optical channel.

On the other hand, the availability of dispersive real-time delay lines is highly desirable in many applications. This component can implement a continuous real time delay in a wavelength range that covers a bandwidth of several optical channels, that is, several hundred GHz. In practice, these dispersive delay lines are implemented by means of components based on fiber, such as for example a coil of several kilometers of dispersive optical fibers, or using Bragg gratings in optical fiber with a pulsed modulated linear frequency. Despite this, neither of these two solutions is prone to their integration into a photonic chip.

Another current example are the devices described in document CN102749782A. This document refers to a method to increase the bandwidth of a delay implemented through a linear offset with the frequency, where the broadband signal is divided, by means of a "demultiplexer", into several channels that need to be equidistant in frequency and, in each channel, an operational linear phase response is implemented for the entire channel bandwidth, which implies an extension of the phase characteristic of the immediately preceding channel; with this, when joining the channels again, by means of a "multiplexer", a linear phase curve is constructed in a bandwidth corresponding to the number of channels.

The proposed method in CN102749782A is designed to delay absolutely the entire spectrum of each of the different channels, this implies that in those embodiments where the separation between optical carrier and radio frequency subcarrier is approximately 30-60 GHz this technique would not be efficient due to to the restriction of the product bandwidth with respect to the delay time. Additionally, this method is designed to implement the same delay in all channels.

DESCRIPTION OF THE INVENTION

A first aspect of the invention describes a tunable optical delay line that can be integrated into an optical chip and configured to implement a complex group delay of optical channels versus optical frequency profiles over wide-band optical spectral regions spanning several hundred of GHz.

A second aspect of the invention describes the method for tuning the delay of the tunable optical delay line of the first aspect of the invention.

More specifically, the tunable optical delay line comprises:

- an input unit for receiving a first optical signal, wherein the first optical signal in turn comprises an optical radiation with a carrier optical frequency wc, and a sideband with a central frequency QRF, wherein additionally said first optical signal is divisible into a plurality of optical channels each defined by a wavelength,

- an optical delay unit SCT linked to the input unit for processing the first optical signal and obtaining a second optical signal, and

- an output unit linked to the optical delay unit for receiving the second optical signal,

- wherein the optical delay unit comprises an adjustment element for applying a specific delay ^{T g} for each optical channel, and said delay ^{T g} is proportional to the interpolation of the values of the carrier optical frequency wc for each channel.

Preferably, the input unit is selected from a 1xN coupler, a demultiplexer, a spectral filter, or a waveguide grating array, and the output unit is selected from a Nx1 coupler, a multiplexer, a spectral filter, or a waveguide grating array.

Preferably, the adjustment element is configured to apply the delay ^{T g,} specific for each optical channel, by means of the SCT technique (separate carrier tuning) adapted to each specific optical channel.

Preferably, the adjustment element comprises at least one optical resonator selected from: ring resonators, micro-ring resonators or free space resonators.

More specifically, the adjustment element comprises a group of four optical resonators for each optical channel to be delayed, where these four optical resonators are configured in a SCISSOR structure (side-coupled integrated sensors of resonators) and apply the delay ^{T g,} being this specific delay ^{T g} for each optical channel.

Preferably, the groups of four optical resonators are structured in parallel.

Alternatively, the groups of four optical resonators are structured in series.

Preferably, the present tunable optical delay line has a tunable optical dispersive delay line architecture called TODDL (Tunable Optical Dispersive Delay Line).

On the other hand, the method for tuning a delay ^Tg in an optical delay line such as that of the first aspect of the invention, comprises:

• set the specific delay ^{T g} for each optical channel, where said delay

^{T g} is proportional to the interpolation of the values of the carrier optical frequency wc for each channel to be delayed.

This method additionally comprises:

• delay the bandwidth associated with the side band with center frequency

^{^ RF,} and

• Offset the carrier optical frequency wc.

This method additionally comprises the following steps to tune the delay:

• set the total delay D1,

• establish a linear slope of the total delay D1,

• set the desired total delay D2,

• modify the linear slope from D1 to D2, and

• individually set desired delays ^{T g} for each optical channel separately and independently according to the modified linear slope.

In this way a group delay characteristic dependent on the linear (ie, dispersive) frequency is obtained, which extends along the multiple channels, giving rise to a "dispersive optical delay line." When implementing said SChT methodology, notes that the spectrum between the optical channels is really zero, therefore, we can dispense with the spectral regions where there are no optical carriers and RF subcarriers, and implement the dispersive delay required by the interpolation separated from the local delays in each channel optical.

Due to this, it is possible within each channel to delay the reduced bandwidth associated with the RF subcarrier while we offset the optical carrier, that is to say that within each channel the SCT technique is used, unlike the previous art where it is necessary to get the delay for each and every channel.

Thus, the present invention does not require that the optical channels be equispaced in frequency or in wavelength.

The present invention can implement linear delays with frequency, while the one proposed in C102749782A is intended to implement the same delay in all channels.

Additionally, the TODDL (Tunable Optical Dispersive Delay Line) architecture provides the following advantages to the present invention:

a) can support broadband and multichannel operation (ie, wavelength division multiplexing, WDM).

b) is composed of elements that can be integrated into a chip and, therefore, is fully integrable in a photonic chip.

c) it can function as a line of dispersive delay of length without the need to resort to fingerprints in the range of cm, m or km.

d) simultaneously exhibits unique characteristics of long delays in combination with a compact photon chip size.

e) it is a passive element and, therefore, can be integrated into the main material forms, such as silicon on insulator (SOl) and silicon nitride (SiNx).

f) Can be programmed to provide linear dispersion profiles vs. frequency and more complex.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization thereof, a set of drawings is attached as an integral part of said description. where, with illustrative and non-limiting character, the following has been represented:

Figure 1a.- Shows a graph showing the principle of the technique of separate tuning of SCT channels that is applied to each specific optical channel.

Figure 1b.- Shows a graph of the separate channel tuning technique called SChT which consists of applying a SCT technique of different values in each optical channel.

Figures from 2a to 2c.- They show illustrations corresponding to possible SChT implementations in parallel.

Figure 3.- They show illustrations corresponding to possible SChT implementations in series.

Figure 4 shows a graphic representation of the desired group delay versus the optical frequency of a linear RF optical delay line. The empty spectral regions are clearly marked and a detail of the effect achieved is shown.

Figure 5.- It shows a graph showing the delay tuning.

PREFERRED EMBODIMENT OF THE INVENTION

The object of the invention is based on a tunable optical delay line that can be integrated into a chip and implement an arbitrary complex group delay versus optical frequency profiles over broadband optical spectral regions spanning several hundred GHz.

The invention is based on the implementation of a broadband tunable optical dispersive delay line by separate interpolation of the desired local group delay characteristic in each optical channel. The local group delay characteristic, ie the delay within the bandwidth of each optical channel, is implemented by a novel principle based on a separate channel tuning technique (SCT) known in the state of the art.

Figure 1a shows the current SCT technique, where the spectral responses in the channel of the real time delay line and the responses of the phase shifter of the carrier and the phase parameter adjustment can be seen. Whereas in Figure 1b the almost constant real-time delay line obtained from the optical carrier channel can be seen.

More specifically, this tunable optical delay line (1) comprising:

• an input unit for receiving a first optical signal, wherein the first optical signal in turn comprises an optical radiation with a carrier optical frequency wc, and a sideband with a central frequency QRF, wherein additionally said first optical signal is divisible into a plurality of optical channels each defined by a wavelength,

• an optical delay unit linked to the input unit for treating the first optical signal and obtaining a second optical signal, and

• an output unit linked to the optical delay unit to receive the second optical signal,

wherein the optical delay unit comprises an adjustment element for applying a specific delay ^{T g} for each optical channel, and said delay ^{T g} is proportional to the interpolation of the values of the carrier optical frequency wc for each channel.

In a preferred embodiment, as shown in 2a, the tunable optical dispersive delay line (1) has a parallel configuration using spacers

^{(split te rs)} of power 1xN (2a) and Nx1 (2b).

This is a simple and lower cost configuration, but since delay units are used such as modules that comprise tunable optical delay lines (OTDL) with independent carrier adjustment (SCT) - hereinafter SCT OTDL module (3), they present a loss of 1 / N power inherent in the Nx1 splitter.

Preferably, an OTDL SCT module (3) is used for each channel of the tunable optical delay line (1). Where each SCT OTDL module (3) comprises its carrier optical frequency wc, the sideband with a central frequency QRF, and its delay ^{T g,}

In another preferred embodiment, as shown in 2b, the tunable optical dispersive delay line (1) has a parallel configuration using AWG (2c).

In the case of using an AWG (2c), the structure is more scalable, since the losses do not increase with the number of SCT OTDL modules (3). In the latter case, the FSR (free spectral range) of the OTDL SCT modules (3) must be greater than that of the AWG (2c). The value for QRF in all OTDL SCT modules (3) is the same and the delays for linear operation are designed as follows.

Said SCT OTDL modules (3) comprise, as shown in Figure 2c, four optical resonators are (4) configured in a SCISSOR structure.

Alternatively, as shown in Figure 3, the SCT OTDL (3) modules, or its variant formed by four optical resonators (4), is connected in series.

The design of the group delay values of each unit depends on the dispersion profile to be implemented. The simplest case, as shown in Figure 4, is when we need a linear group delay characterized by a slope or dispersion parameter given by D. In this case, after setting the value for the first unit Tg1 the group delay for module ^{SCTOTDL k, T gk ,,} It can be found in terms of that of unit k-1:

Which derives in:

^{T g, k = T g, l D (& c, k} -® c, 0 '

In those embodiments in which the units having at least one tunable channel separator element (SChT) are uniformly spaced (equidistant), the equation becomes:

^{T g, k = T g, 1 k D} A® ■

In those embodiments in which there are more complex or arbitrary group delay profiles, it is necessary to know the value of the local dispersion parameter, that is, if Dkk.1 is the local dispersion parameter, then it applies:

Resulting in:

^{Tg, k = Tg, 1} + Z ^{, r} [ ^{D r, r - l - D r + 1, r} ] ^{, kD k, kl ~ ®c, 1 D 21} ■

r = 2

Preferably, as shown in Figure 5, to proceed to configure the programming of the tunable optical dispersive delay line (1) of the invention only requires the change of the slope of the total group delay line from a value initial D1 to a final value D2 and then individually reprogram the delays of each SCT OTDL module (3) separately and independently through the phase shifters in their annular cavities. For a linear group delay (TODDL), this involves changing the individual delays of

Claims (9)

1. Tunable optical delay line (1) which is configured to be integrated in an optical chip and which comprises: • an input unit for receiving a first optical signal, wherein the first optical signal in turn comprises an optical radiation with a carrier optical frequency wc, and a sideband with a central frequency QRF, wherein additionally said first optical signal is divisible into a plurality of optical channels each defined by a wavelength, • an optical delay unit linked to the input unit for treating the first optical signal and obtaining a second optical signal, and • an output unit linked to the optical delay unit to receive the second optical signal, wherein the tunable optical delay line (1) is characterized in that the optical delay unit comprises an adjustment element for applying a specific delay ^{T g} for each optical channel, and said delay ^{T g} is proportional to the interpolation of the values of the carrier frequency wc for each channel. 2. Tunable optical delay line (1), according to claim 1, characterized in that the adjustment element is configured to apply the delay Tg, specific for each optical channel, by means of the SCT technique (tuning of separate channels). 3. Tunable optical delay line (1) according to any one of the preceding claims, characterized in that the adjustment element comprises at least one optical resonator selected from: ring resonators, micro-ring resonators or free space resonators. 4. Tunable optical delay line (1), according to claim 3, characterized in that the adjustment element comprises a group of four optical resonators for each optical channel to be delayed, wherein these four optical resonators (4) are configured in a SCISSOR structure (side-coupled integrated squecen of resonators) and apply the delay Tg, this delay being specific ^{T g} for each optical channel. 5. Tunable optical delay line (1), according to claim 4, characterized in that the groups of four optical resonators (4) are structured in parallel. 6. Tunable optical delay line (1), according to claim 4, characterized in that the groups of four optical resonators (4) are structured in series. 7. Method for tuning a delay ^{T g} in a tunable optical delay line (1) as described in any one of the preceding claims, wherein the method is characterized in that it comprises: • Set the specific delay ^{T g} for each optical channel, where said delay ^{T g} is proportional to the interpolation of the values of the carrier optical frequency wc for each channel to be delayed. 8. Method according to claim 7, characterized in that additionally, it comprises: • delay the bandwidth associated with the side band with center frequency ^{^ RF,} and • Offset the carrier optical frequency wc. 9. Method according to claim 7, characterized in that additionally, it comprises: • set the total delay D1, • establish a linear slope of the total delay D1, • set the desired total delay D2, • modify the linear slope from D1 to D2, and • individually set desired delays ^{T 'g} for each optical channel separately and independently according to the modified linear slope.