ES2216689A1 - Group delay measuring method, involves generating optical frequency phase difference between sidebands by amplitude modulation of optical carrier using sinusoidal electrical signal for modulating optical signal - Google Patents

Abstract

The method involves generating optical frequency phase difference between sidebands by amplitude modulation of an optical carrier using a sinusoidal electrical signal for modulating an optical signal. Detected electrical signal with double frequency is compared to the frequency of the modulating signal. Spectral curve of group delay is determined from frequency sweep of the optical carrier through a spectral purity optic or similarly tunable maintaining fixed frequency of the modulating signal.

Description

Method to measure the frequency response of group delay eliminating errors due to variations frequencies in the amplitude response.

Technical sector

Optical communications.

Background of the invention

The high demand for connectivity and the increasing competitiveness among companies providing telecommunication services, which requires the systems of better and better performance transmission with better efficiency, both in terms of the economic cost of its construction and maintenance, as of time for commissioning, are undergoing a hard test the possibilities of technology current that must evolve by leaps and bounds to keep the pace of growth of a new society based on the possibilities offered by remote communication.

One of the fronts on which the battle is going revealing more difficult is the combat of the chromatic dispersion on long distance links using fiber optics, especially in submarine links, for its unique limited accessibility features once built, and hostile marine environment conditions.

Numerous techniques are emerging to mitigate the devastating effects on signal quality. Common to all they are the need to know exactly and reliably the characteristics of the chromatic dispersion (the derivative with respect to the wavelength of the group delay) introduced by both the fiber as per devices designed to compensate for its effect degrading on the signal. In this sense, the impact stands out on the quality of long distance transmissions produced by micro-fluctuations in the spectral curve of group delay presented by Bragg diffraction networks in fiber (FBG, hereinafter) and that are due to inaccuracies inherent in current manufacturing methods.

On the other hand, the introduction of techniques WDM and WDM dense, multiplexing channels using various wavelengths with reduced spectral spacing (techniques current point to the possibility of transmitting on a single fiber several hundreds of channels with spacing of few tens GHz), will impose very strict requirements on characterization and the design of multiplexing, demultiplexing devices, filtered, routed etc. (many of them based on FBGs) with regard especially to the spectral resolution, the resolution of the measures and the simplification and ease of automation of the necessary processes and assemblies for your industrial application in mass production chains of photonic devices

The method that is considered standard in measures of dispersion and / or group delay has appeared described as a method of modulation and phase change, "Modulation Phase Shift Method ", (S. Ryu, Y. Horiuchi and K. Mochizuki," Novel Chromatic Dispersion Measurement Method over Continous Gigahertz Tuning Range ", J. of Lightwave Technol., Vol. 7, No. 8, 1989; registered with patent number US4984884) and we will refer to it in forward, as is usually done in the literature, such as MPSM. The method is based on determining the group delay that the device to be measured (DUT, hereinafter) introduces to each length of wave. This group delay is obtained from the difference measurement between the phases that the DUT introduces to each of the bands laterals generated by the amplitude modulation of the carrier optics using a pure sinusoidal frequency electrical signal fixed. From there, differentiating from the length of wave, the value of the dispersion can be obtained. To obtain the group delay spectral curve, the optical carrier leaves sweeping along the band of optical frequencies of interest by means of a monochromatic optical source of high spectral purity (a laser) tunable.

This method is useful when the answer in DUT amplitude is essentially flat, since in the presence of Significant amplitude variations in the resolution range in wavelength of the measurement, given by double the frequency of the electrical signal that modulates in amplitude the optical carrier, the phase difference obtained depends on significant way of the ratio of signal amplitudes to each of the side bands.

In order to adapt this method to the increasing demands of the industry, have been appearing modifications that improve its performance, some adapted to the specific characteristics of some applications, such as phase error cancellation method, described in (Y. Horiuchi, Y. Namihira and H. Wakabayashi, "Chromatic Dispersion Measurements of 4564Km Optical Amplifier Repeater System ", Electronic Letters, Vol. 19, No. 1, 1993), and methods that combine each other or cancel any of the sidebands generated in the intensity modulation process of the optical carrier for improve wavelength resolution, such as those described in (J. E. Román, M.Y. Frankel and R. D. Esman, "Spectral Characterization of Fiber Gratings with High Resolution ", Opt. Lett., Vol. 23, No. 12, June 1998; R. Fontenberry, "Enhanced Wavelength Resolution Chromatic Dispersion Measurements using Fixed Sideband Technique ", Technical Digest OFC'98, pp. 363-364, 1998, with patent No. US6088088; C. K. Madsen. "Chromatic and Polarization Mode Dispersion Measurements using Phase-sensitive Sideband Detection ", Technical Digest OFC'01, pp. MO6-1 / -3, 2001), among others.

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Although other methods of measurement of dispersion also based on amplitude modulation by a pure sinusoidal electrical signal but not in phase detection imposed between the sidebands but in the interference between them, in which the measure of dispersion is, as in the invention presented here, independent of the breadth of the DUT response to each of the bands (C. Peucheret, F. Liu and R. J. S. Pedersen, "Measurement of Small Dispersion Values in Optical Components ", Electronics Letters, Vol. 35, No. 5, March 1999); the levels of spectral resolution and the measurement that they get are lower than those of the MPSM and therefore those of the method described here. On the other hand, they require adaptation from the measurement conditions to the expected value of the dispersion in a difficult assembly to standardize and automate, and necessarily slow, so it is difficult to adapt to the characterization of devices in industrial environments with mass production processes.

So far, the authors do not have constancy of any method that essentially on an assembly identical to the standard method, which ensures a resolution of the measurement and wavelength, and a speed in obtaining  results and an automation facility, appropriate to the characterization of optical devices in industrial processes of mass production, eliminate the error caused by fluctuations in the frequency response of amplitude of the DUT.

Description of the invention

The present invention relates to methods of characterization of the spectral response of photonic devices such as fibers or diffraction networks of Bragg, especially those that use amplitude modulation of a signal optics using a radio frequency signal generating a signal optics whose spectrum contains both side bands on both sides of the carrier and that is injected into the DUT for the analysis of the spectral response of this, see Fig. 2. It basically consists of a modification of the standard method of group delay measurement on photonic devices intended to eliminate the error due to fluctuations in the spectral response of significant amplitude in the spectral resolution range of the measure.

When the amplitudes of the two bands can considered approximately equal, the phase of the detected signal at the frequency f_ {m} relative to the phase of the modulating signal provides the semi-difference between the phases that the device to measure (DUT) imposes between the bands, \ frac {\ phi ^ {+} - \ phi ^ {-}} {2} = \ frac {\ Delta \ phi} {2}. Assuming the group delay, \ tau_ {g} = d \ phi / d \ omega, can be considered approximately constant in the frequency range delimited by the two bands, their value can be obtained by

(1) \ tau_ {g} \ approx \ frac {\ Delta \ phi} {2 \ omega_ {m}}

However, in the case that the amplitudes of each of the bands are significantly different, i.e. A +, A -, the phase of the signal detected at the frequency f_ {m} with respect to the phase of the modulating signal corresponds to the expression

(2) \ frac {\ Delta \ phi + \ delta} {2} + tg ^ {- 1} \ left (\ frac {sin (\ delta)} {A + {/} -} -cos (\ delta)} \ right)

with \ delta = \ phi + {+} - \ - 2 \ phi_ {0} and \ phi_ {0} the phase that the optical device imposes on carrier frequency The error introduced to the extent of the phase half difference between the bands in case there is a significant difference between the amplitudes at each of the bands is the same to

(3) \ varepsilon = \ frac {\ delta} {2} + tg ^ {- 1} \ left (\ frac {sin (\ delta)} {A + {/ A ^ {-} - cos (\ delta)} \ right)

This error is not easily eliminated from the measure, even if the frequency response of amplitude of the DUT, that is, the value of A + / A -, because it depends in any case on the value of δ, which in turn, comes from the sum of the phases, quantities that precisely pursues determine.

The method we propose here provides a way to obtain the phase difference that the DUT imposes between bands, \ Delta \ phi, avoiding the error given by (3) and without just changes in experimental setup, taking advantage of the fact that the process of returning the signal to the electrical domain is carried out by direct detection of the optical signal at the output of the DUT. The direct detection process has a characteristic of optical-electrical conversion of quadratic law, by which in addition to the signal at the frequency f_ {m}, result of the mixing each of the sidebands with the frequency carrier; as a result of the mixture of the two bands between yes, a signal at frequency 2f_ {m} is obtained, see Figure 2.

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The phase of the signal detected at 2f_m with with respect to the second harmonic of the modulating signal provides the phase difference between the bands, Δ \ phi, regardless of the value of the amplitude of the optical signal at the DUT output for each of the sidebands. Is phase difference can be measured by an Analyzer Standard Microwave Transitions and hence get the delay of group according to the expression (1).

Thus, as an additional precaution regarding MPSM for the proper functioning of the method presented here, make sure that the bandwidth of the optical detector, as well as of the elements that send the signal to the Analyzer Transitions, exceeds by a reasonable percentage twice the modulating frequency This is not a very important restriction.  since to ensure good spectral resolution of the measures the modulating frequency is usually set at around 100MHz., with what is not expected that the bandwidth required in the detector significantly increases the mounting.

Application of the invention

The method described here allows to know the group delay response of photonic devices with higher fidelity to the standard MPSM method in cases where the amplitude response present significant variations in the Wavelength resolution range of the delay measurement of group.

An accurate knowledge of this answer results fundamental in the characterization of transmission systems by fiber optic, especially in long distance and transoceanic, with respect to chromatic dispersion. East phenomenon, responsible for the temporary widening of the pulses that are transmitted through the system, reduces the speed of data transmission and therefore the transmission capacity of the link. An exact characterization of the chromatic dispersion that introduces fiber and other transmission devices is a key aspect, not only for the evaluation of benefits of the link, but in the design and practical realization of photonic devices that have group delay curves that adapt to the system dispersion conditions, as per example FBGs and dispersion compensating fibers, and allow their compensation, thus increasing the transmission capacity of the link (Y. Danziger and D. Askegard, "Full-band Chromatic-dispersion Management Improves Performance "Lightwave, Penn Well Corporation, July 2000).

In the case of FBGs, the characterization precise of your group delay curve in a simple and easily standardizable and automatable, it becomes a issue of special interest because its applications go beyond the compensation of chromatic dispersion in long bonds distance by fiber having increased its production so considerable in recent years. They are also fundamental in the filtering, multiplexing, demultiplexing and signal routing in WDM wavelength multiplexing systems (R. Kashyap, "Fiber Bragg Gratings", Academic Press, San Diego, 1999), in which the amount of channels and spectral separations typical among them with which it is expected to transmit demand large spectral resolutions. On the other hand, since the distances to cover are large and therefore the number of cascaded devices, precision measurements are it also becomes a key factor (X. Zheng and F. Liu, " Impact from Amplitude Ripples in Fiber Bragg Grating Optical Add-drop Multiplexers and Cross-connects ", 27th European Conference on Optical Communication ECOC 2001).

FBGs also find a vast field of application as sensors (R. Kashyap, "Fiber Bragg Gratings", Academic Press, San Diego, 1999), for example tension sensors distributed (S.H. Huang et al. Conf. Smart Sensing, Process. and Instrum SPIE 2444, pp 158-169, 1995).

Brief description of the drawings

Figure 1 shows a preferred embodiment of the group delay measurement assembly according to the method object of this invention.

Key:

(one)

device a measure (DUT)

(two)

Electrical signal modulator: A_ {m} cos (\ omega_ {m} t + \ theta_ {m})

(3)

optical signal monochromatic used as carrier: A_ {o} cos (\ omega_ {o} t  + \ theta_ {o})

(4)

To be tunable

(5)

Analyzer Transitions

(6)

Optical power detected from the optical response of the DUT, P_ {D}

(7)

Optical modulator of amplitude

(8)

Optical signal amplitude modulated, S_ {M}

(9)

Optical detector (of quadratic law)

(10)

Optical signal DUT response, S_ {G}

Figure 2 graphically shows the composition spectral of the optical signal injected into the DUT with indication of the amplitude and phase that each spectral component acquires by DUT action and frequency mixing processes that have place as a result of the quadratic law detection process.

Description of a preferred embodiment

The measurement assembly of the chromatic dispersion which is proposed as a preferred embodiment of the method here Presented is shown in Figure 1 and consists of a wave laser variable wavelength continuous (4) responsible for sweeping sequentially the optical wavelength in the range of frequencies of interest The laser is synchronized with a Standard Microwave Transition Analyzer (5) so that each emission wavelength of the laser is made correspond the measurement made in (6). The optical output signal of the laser is amplitude modulated by a modulator Mach Zehnder type electro-optic (7) that injects the optical signal, already modulated (8) by a frequency sinusoidal electrical signal fixed, f_ {m} (2) in the DUT (1).

The response of the device (10), in reflection In the case we consider here, it is detected by a detector optical (9) and injected into the Transition Analyzer Microwave, which is able to obtain the phase difference between the signal detected at the frequency 2f_m with respect to the second harmonic of the modulating signal and from that result, the group delay by expression (1).

In order to obtain good spectral resolution of the measurements, the modulation frequency f_ {m} is usually set around 100MHz, which at the wavelength of 1.55 \ mum assumes a resolution of about 1pm. Therefore the optical detector (9) should have a minimum bandwidth somewhat greater than 200 MHz.

Claims (2)

1. Measurement method of the group delay at optical frequencies characterized in that the phase difference between the lateral bands generated by amplitude modulation of the optical carrier using a sinusoidal electrical signal as a modulator, of the optical response signal of a device to be measured , is obtained from the electrical signal detected at double frequency with respect to the frequency of the modulating signal. 2. Group delay measurement method according to claim 1, characterized in that the group delay spectral curve is determined from the frequency scanning of the carrier optical signal by means of a tunable or similar high spectral (laser) optical source, maintaining set the frequency of the electrical modulator signal.