Optical amplification system

Abstract

Title: Optical Amplification System The invention relates to an optical amplification system (100), comprising: a semiconductor optical amplifier (1) configured to allow, under predetermined operating conditions, the appearance of amplitude gain ripples (deltaG) greater than 1 dB, in particular greater than 2 dB or 3 dB, the optical amplifier being configured to amplify a probe signal to at least one wavelength chosen within a period of the gain ripple to allow a modulation bandwidth of the probe signal by the optical amplifier that is greater than or equal to 5 GHz, in particular being substantially equal to 10 GHz. Figure for the abstract: Fig. 1

Description

Title of the invention: Optical amplification system

[0001] The present invention relates to an optical amplification system.

[0002] Semiconductor optical amplifiers (SOAs) have limited electrical bandwidth, typically on the order of 2 to 5 GHz under nominal operating conditions. This reduces the data transmission capacity on a channel using cross-gain modulation (XGM), thus constituting a major obstacle to optimizing the performance of optical communication systems.

[0003] Existing solutions requiring additional components to the SOA can increase the electrical bandwidth of the SOA, such as optical filters. This complicates the system.

[0004] The present invention aims in particular to overcome these defects.

[0005] The invention thus relates to an optical amplification system, comprising: - a semiconductor optical amplifier configured to allow, under predetermined operating conditions, the appearance of gain ripples (in English, Ripple) of amplitude (deltaG) greater than 1 dB, in particular greater than 2 dB or 3 dB, - the optical amplifier being configured to amplify a probe signal (also called probe laser) to at least one wavelength chosen within a gain ripple period to allow a modulation bandwidth of the probe signal by the optical amplifier which is greater than or equal to 5 GHz, and in particular substantially equal to 10 GHz

[0006] If desired, the invention can allow a modulation bandwidth of the probe signal which is greater than or equal to 20 GHz, in particular for a gain which is greater than 45 dB (this level of gain not necessarily being mandatory).

[0007] According to one aspect of the invention, the optical amplifier is configured to allow optical-to-optical modulation, namely modulation of the probe signal (also called the probe laser) by a pump signal (also called the pump laser). In the optical amplifier, a portion of the energy from the pump laser is transferred to the probe laser.

[0008] The probe laser and the pump laser may have different wavelengths or, alternatively, substantially the same wavelength. If the wavelengths are close, it is necessary to ensure that they can be separated by optical filtering. If the wavelengths are identical, polarization control of the signals can be used, for example, orthogonal polarization, or contradirectional injection of the pump and the probe.

[0009] According to one aspect of the invention, upstream of the optical amplifier, the probe laser and the pump laser are added to each other by an optical coupler of the system.

[0010] When the probe laser is modulated around its optical carrier, modulation lines appear, and the invention uses a choice of a wavelength line, in a period of the gain ripple, which makes it possible to widen the bandwidth.

[0011] According to one aspect of the invention, a line (RS) of the probe signal is placed to the right (i.e. in the direction of the longest wavelengths) of a peak (Pk), in particular substantially in the middle of a descent of the gain ripple, and the line (RP) of the pump signal is placed substantially in a trough of the gain ripple.

[0012] In other words, in the invention, the inventors unexpectedly discovered that by introducing gain ripples, the modulation bandwidth is increased (for example, by tripling it), provided that the ripples are positioned correctly within the spectrum. Thus, a wavelength (modulo the ripple period) is chosen to maximize the bandwidth, and this choice is made within a gain ripple period.

[0013] Further explanations on these aspects are given below.

[0014] According to one aspect of the invention, an optical signal input interface and an optical signal output interface of the optical amplifier have a reflection coefficient which, at an operating temperature of the optical amplifier, is adapted to allow gain ripples to appear.

[0015] In one embodiment of the invention, the optical amplifier is configured to operate at a cryogenic temperature level, in particular a temperature between 30 K (Kelvins) and 80 K, for example equal to 55 K.

[0016] At this cryogenic temperature, the optical amplifier becomes extremely efficient, and a significantly higher gain is achieved than at room temperature. The gain can increase to such an extent that cavity effects appear, particularly at temperatures below 150 K, for example, 77 K or 134 K. This results in gain ripples that can reach, for example, 7 dB. This is because the increase in gain makes anti-reflective surfaces less suitable for these temperatures. Lowering the temperature degrades the effect of the anti-reflective coating relative to the gain. The anti-reflective coating is less effective for three main reasons: - the increased optical gain requires less parasitic reflection, for the same ripple. - The characteristics of the treatment are influenced by the cold, - as the gain spectrum shifts in wavelength, the processing is no longer well adapted to the wavelengths used.

[0017] Thus the inventors obtain a beneficial effect by going against the usual common sense which is to keep anti-reflective treatments effective.

[0018] In another embodiment of the invention (which does not use cryogenic temperatures), the reflection coefficients RI and R2 are chosen to be higher than the values ​​usually chosen to minimize gain ripple, to accommodate the moderate gain at room temperature and thus obtain an adequate ripple value. In this case, the optical amplifier can be used at room temperature. For example, an optical signal input interface and an optical signal output interface of the optical amplifier have reflection coefficients RI and R2 that are on the order of, or less than, 10⁴ or 10⁵.

[0019] In particular, the adjustment of the reflectivity is obtained through the manufacturing process of the component (the optical amplifier).

[0020] In the invention, it is the combination of two factors, namely the gain and the reflection coefficients, that allows for gain ripples.

[0021] According to one aspect of the invention, the system includes a temperature sensor and a temperature control module to regulate the temperature of the optical amplifier (in particular to cool it).

[0022] The temperature control module is configured to regulate the temperature in the optical amplifier to cryogenic temperature levels.

[0023] According to one aspect of the invention, the optical amplifier is configured to obtain, at cryogenic temperature levels, a bandwidth multiplied by a factor of 2 to 10 compared to ambient temperature, making it possible to achieve bandwidths up to 20 GHz.

[0024] According to one aspect of the invention, the optical amplification band of the semiconductor optical amplifier (which corresponds to wavelength ranges over which the semiconductor optical amplifier is effective at amplifying light signals) can be an S, C, or L band, which are notably wavelength ranges used in optical telecommunications. The S band (short wavelength band) covers wavelengths from 1460 to 1530 nanometers (nm). The C band (conventional wavelength band) covers wavelengths from 1530 to 1565 nm. The L band (long wavelength band) covers wavelengths from 1565 to 1625 nm. The choice of the amplification band depends on the specific component of the semiconductor optical amplifier and the temperature at which it operates.

[0025] According to one aspect of the invention, the optical amplifier is preceded by a polarization controller for each optical signal to control their optical polarization and optimize the gain.

[0026] For example, the polarization controller is configured to generate a polarization to have the probe signal and the pump signal with the same wavelength, and with different polarizations, for example orthogonal polarizations (for example one optical signal has a horizontal polarization and the other optical signal has a vertical polarization).

[0027] According to one aspect of the invention, the optical amplification system according to the invention is used as an optical modulator in a telecommunications network. This makes it possible to increase the data transmission capacity by expanding the bandwidth, for example to reach 10 GHz, for example instead of 3 GHz for known modulators using a semiconductor optical amplifier (SOA).

[0028] The invention also relates to an optical modulator in a telecommunications network comprising an optical amplification system as mentioned above.

[0029] According to one aspect of the invention, the optical amplification system is configured to receive an optical frequency comb and to amplify and modulate each of the frequencies of the optical frequency comb. In the invention, each of the comb frequencies is chosen so that, for each of these comb frequencies, the bandwidth is broadened. This results in improved performance. The optical amplifier can be configured for use in a quantum computer.

[0030] The invention also relates to a quantum computer comprising an optical amplification system as mentioned above.

[0031] Other features, details and advantages of the invention will become clearer upon reading the following description on the one hand, and several illustrative and non-limiting examples of embodiments given with reference to the accompanying schematic drawings on the other hand, in which:

[0032] [Fig-1] Fig.1 is a schematic representation of the amplification system optics according to an example of an embodiment of the invention;

[0033] [Fig.2] Fig.2 represents gain curves obtained with the system of Fig.1

[0034] [Fig.3] Fig.3 illustrates, very schematically, gain ripples obtained with the system of [Fig.1];

[0035] [Fig.4] The [Fig.4] represents lines in the gain ripple, according to the invention, for a quantum computer.

[0036] The features, variants, and different embodiments of the invention can be combined in various ways, provided they are not incompatible or mutually exclusive. In particular, variants of the invention may be conceived comprising only a selection of features, described hereafter in isolation from the other described features, if this selection of features is sufficient to confer a technical advantage and / or to differentiate the invention from the prior art.

[0037] Figure 1 shows an optically amplified system 100 comprising a semiconductor optical amplifier 1 (also called a SOA chip for "Semiconductor Optical Amplifier" in English) configured to allow, under predetermined operating conditions, the appearance of gain ripples (in English, Ripple) with an amplitude (deltaG) greater than 1 dB, in particular greater than 2 dB or 3 dB. The cross-gain modulation technique (also designated by XGM) is used here.

[0038] The optical amplifier 1 is configured to amplify a probe signal PrS (also called probe laser) to at least one wavelength chosen in a period of the gain ripple to allow a modulation bandwidth of the probe signal by the optical amplifier which is greater than or equal to 5 GHz, in particular being substantially equal to 10 GHz.

[0039] The optical amplifier 1 has an optical signal input interface 2, in particular for connection to an input optical fiber, and an optical signal output interface 3, in particular for connection to an output optical fiber. An optical coupler 25 is provided at the input of the solid-state optical amplifier 1 for combining the pump and probe signals. A bias controller 26 for the pump and probe signals is also provided.

[0040] The optical amplifier 1 is configured to allow optical-to-optical modulation, namely modulation of the probe signal (also called the probe laser) by a pump signal (also called the pump laser). In the optical amplifier, a portion of the energy from the pump laser is transferred to the probe laser.

[0041] We thus have a probe-pump assembly.

[0042] The probe laser and the pump laser are at different wavelengths.

[0043] The system 100 includes a modulator 5 configured to receive as input a continuous optical signal which is modulated to form the PuS pump signal or pump laser.

[0044] At the input of the optical amplifier 1, the pump signal PuS has a substantially greater power than the probe signal PrS, which has a relatively low power.

[0045] The pump signal PuS is used to modulate the gain of the optical amplifier 1.

[0046] More precisely, the envelope of the signal modulated by the modulator 5 is configured to modulate the gain of the optical amplifier 1 and the probe signal PrS, which is a continuous signal at the input of the optical amplifier 1, will have the gain modulated by passing through the optical amplifier 1. At the output of the optical amplifier 1, the probe signal PrS comes out amplified and modulated.

[0047] We have thus, in a way, transposed modulation information from the first, high-power signal to the second, low-power signal at the output of the optical amplifier 1.

[0048] Upstream of the optical amplifier, the probe laser and the pump laser are added one on top of the other by an optical coupler of the system.

[0049] The optical signal input interface 2 and the optical signal output interface 3 of the optical amplifier 1 have a reflection coefficient RI, respectively R2, which is, at an operating temperature of the optical amplifier 1, suitable to allow gain ripples to appear.

[0050] In the example described, the optical amplifier 1 is configured to operate at a cryogenic temperature level, in particular a temperature between 30 K (Kelvins) and 80 K, for example equal to 55 K.

[0051] For this purpose, the optical amplifier 1 is placed in a cryostat enclosure 17. The system 100 includes a temperature sensor and a temperature control module to regulate the temperature of the optical amplifier (in particular to cool it). The temperature control module is configured to regulate the temperature in the optical amplifier 1 to cryogenic temperature levels.

[0052] When the optical amplifier 1 is put into a cold state, at cryogenic temperature, a ripple in the gain is observed in its optical bandwidth (see [Fig.3]).

[0053] At this cryogenic temperature, the optical amplifier 1 becomes extremely efficient, and a significantly higher gain is achieved than at room temperature. The gain can increase to such an extent that optical cavity effects appear, particularly at temperatures below 150 K, for example, 77 K or 134 K. Gain ripples of up to 7 dB are then observed. This is because the increase in gain makes the anti-reflective surfaces less suitable at these temperatures. Lowering the temperature degrades the effect of the anti-reflective coating relative to the gain, and this causes oscillations in the wavelength response of the cavity.

[0054] In the mathematical formula below, it can be seen that ripples deltaG can be generated either by lowering the reflection coefficients RI and R2 (by initial construction of the optical amplifier), or by increasing the gain Gs (Single-pass gain in the cavity) by operating the optical amplifier at a cryogenic temperature level:

[0055] AG

[0056] At the output of the optical amplifier 1, the probe signal PrS is sent to a photodiode 8. An optical filter 27 is placed between the optical amplifier 1 and the photodiode 8. The filter is to be tuned to the wavelength of the probe.

[0057] System 1 may include a network analyzer 9 configured to measure the transfer function to determine the electrical bandwidth. The network analyzer 9 may be replaced by a signal generator.

[0058] When the PrS probe laser is modulated around its optical carrier, modulation lines appear, and the invention uses a selection of a wavelength line within a period of the gain 20 ripple (or "Ripple") to broaden the bandwidth. Figure 3 schematically illustrates gain 20 ripples, with the wavelength WL on the x-axis and the gain GN on the y-axis.

[0059] Figure 3 shows an RP line of the pump signal and an RS line of the probe signal. The lines are positioned relative to a peak Pk of the ripples. Peak Pk serves as a reference. It can be seen that the variation in the peak levels is quite slow for a few gain ripple periods around a peak. Thus, either peak can be used interchangeably as the reference.

[0060] The RP line of the pump signal is placed at a certain wavelength relative to the peak Pk, and the RS line of the probe signal is placed at a certain wavelength relative to the peak Pk.

[0061] The RS line of the probe signal is placed to the right of a Pk peak (i.e. in the direction of the longest wavelengths), namely substantially in the middle of a descent of the ripple, and the RP line of the pump signal is placed in a trough.

[0062] Thus, the placement of the lines is oriented to increase the modulation bandwidth.

[0063] Figure 2 shows representative gain curves of what happens through system 100 (with optical amplifier 1 at 55 kΩ and 25 mA applied), in terms of bandwidth. The x-axis represents the frequency in GHz. The y-axis represents the normalized gain in dB.

[0064] It can be seen that, thanks to the configuration illustrated in [Fig. 3], the invention makes it possible to greatly increase the bandwidth, up to approximately 17 GHz (see curve C), compared to cases where the RS line of the probe signal is placed in a trough (curve Ca), at a peak (curve Cb), or to the left of a peak Pk (curve Ce), that is to say in the direction of the shortest wavelengths.

[0065] The invention makes it possible to have a compact system (with a single component).

[0066] In one embodiment, the reflection coefficients RI and R2 (by initial construction of the optical amplifier) ​​are on the order of, or less than, 10⁴ or 10⁵. In this case, the optical amplifier can be used at room temperature.

[0067] In particular, the adjustment of the reflectivity is obtained through the manufacturing process of the component (the optical amplifier).

[0068] In the invention, it is the combination of two factors, namely the gain and the reflection coefficients, that allows for gain ripples.

[0069] According to one aspect of the invention, the optical amplifier is preceded by a polarization controller for each optical signal to control their optical polarization and optimize the gain.

[0070] For example, the polarization controller is configured to generate a polarization to have the probe signal and the pump signal with the same wavelength, and with different polarizations, for example orthogonal polarizations (for example one optical signal has a horizontal polarization and the other optical signal has a vertical polarization).

[0071] For a quantum computer application illustrated in [Fig. 4], a frequency comb can be amplified where the distance between each line is a multiple of the gain ripple period. By placing these lines in the region to the right of the ripple (i.e., in the direction of longer wavelengths), between a peak and a trough, the bandwidth of each line can be maximized.

Claims

Demands

1. Optical amplification system (100) comprising: - a solid-state optical amplifier (1) configured to, under predetermined operating conditions, allow the appearance of amplitude gain ripples (deltaG) greater than 1 dB, in particular greater than 2 dB or 3 dB, - the optical amplifier being configured to amplify a probe signal to at least one wavelength chosen in a period of the gain ripple to allow a modulation bandwidth of the probe signal by the optical amplifier which is greater than or equal to 5 GHz, in particular being substantially equal to 10 GHz.

2. System according to the preceding claim, wherein the optical amplifier (1) is configured to permit optical-to-optical modulation, namely modulation of the probe signal by a pump signal.

3. System according to the preceding claim, wherein a line (RS) of the probe signal is placed to the right of a peak (Pk), in the direction of the longer wavelengths, in particular substantially in the middle of a descent of the gain ripple, and the line (RP) of the pump signal is placed substantially in a trough of the gain ripple.

4. System according to any one of the preceding claims, wherein an optical signal input interface (2) and an optical signal output interface (3) of the optical amplifier have a reflection coefficient which, at an operating temperature of the optical amplifier, is adapted to allow gain ripples to appear.

5. System according to the preceding claim, wherein the optical amplifier (1) is configured to operate at a cryogenic temperature level, in particular a temperature between 30 K (Kelvins) and 80 K, being for example equal to 55 K.

6. System according to claim 4, wherein an optical signal input interface (2) and an optical signal output interface (3) of the optical amplifier have reflection coefficients RI and R2 which are on the order of, or less than, 10⁴ or 10⁵.

7. System according to any one of the preceding claims, wherein the system comprises a temperature sensor and a temperature control module for regulating the temperature of the optical amplifier.

8. System according to any one of the preceding claims, wherein the optical amplifier (1) is preceded by a polarization controller for each optical signal to control their optical polarization and optimize the gain.

9. System according to any one of the preceding claims, wherein the optical amplification system (100) is used as an optical modulator in a telecommunications network.

10. A system according to any one of claims 1 to 8, wherein the optical amplifier is configured to receive an optical frequency comb, and amplify and modulate each of the frequencies of the optical frequency comb.

11. Optical modulator in a telecommunications network comprising an optical amplification system according to any one of claims 1 to 9.

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