This paper reports on the experimental implementation of an interferometer featuring sum frequency generation (SFG) processes powered by a pump spectral doublet. The aim of this configuration is to allow the use of the SFG process over an enlarged spectral domain. By analyzing the converted signal, we experimentally demonstrate a frequency spectral compression effect from the infrared input signal to the visible one converted through the SFG process. Recently, such a compression effect has been numerically demonstrated by Wabnitz et al. We also verify experimentally that we fully retrieve the temporal coherence properties of the infrared input signal in the visible field. The experimental setup permits to demonstrate an experimental frequency spectral compression factor greater than 4. This study takes place in the general field of coherence analysis through second order non-linear processes.
© 2013 Optical Society of America
Frequency conversion has been extensively used in the field of radio and microwave applications for a long time. This operation has been also achieved in the optical domain thanks to various nonlinear parametric interactions such as second harmonic, difference and sum frequency generation (SFG) in a nonlinear crystal or waveguide. In particular, SFG in a periodically poled Lithium Niobate (PPLN) crystal or waveguide allowed to convert an infrared wave into a visible one by using a pump source as shown in many applications [1–4].
In the field of high resolution imaging, we have recently demonstrated the ability to analyze the coherence properties of an infrared source through an up-conversion process in a two and a three-arm interferometer [5,6]. All over these first experiments, we were limited by the intrinsic spectral acceptance of the PPLN waveguides: if a large spectral bandwidth source at infrared wavelengths is converted through a SFG process with a single line pump, the PPLN spectral acceptance limits the nonlinear effect to a very narrow spectral bandwidth of the source. To overcome this spectral limitation, several techniques have already been proposed, involving an up-conversion detector which samples  or scans  the input signal spectrum by using a tunable pump laser.
As a long term goal we intend to study a SFG process powered by a pump frequency comb to increase the spectral bandwidth to be analyzed while preserving signal coherence information.
Wabnitz et al. have recently demonstrated numerically that the use of a non-monochromatic pump source to power a SFG process, converting a large spectral bandwidth infrared signal, leads to a frequency spectral compression of the converted wave through the non-linear process .
We intend here to experimentally demonstrate this frequency spectral compression behaviour through a SFG process. We use a pump spectral doublet to convert an input spectral doublet. In this configuration, each line of the pump spectral doublet addresses a single line of the input spectral doublet. Through the signal temporal coherence analysis, we experimentally demonstrate the frequency spectral compression effect and we verify that the temporal coherence information of the infrared signal is fully retrieved in the converted visible field.
2. Simulation results
The sum frequency generation is a non-linear process where a signal and a pump wave interact in a non-linear medium to generate a converted wave . If νs and νp are the frequencies of the signal and pump waves respectively, the frequency of the converted wave is inferred from the energy conservation law: νc = νs + νp. The conversion efficiency strongly depends on the phase mismatch between the three interacting waves. In a PPLN waveguide, this phase mismatch is given byFigure 1 shows a color-map of the normalized conversion efficiency for any signal/pump couple around the mean pump frequency THz ( nm) and the mean signal frequency THz ( nm).
It is worth noting that the converted frequency is not represented here, but can be inferred from the energy conservation law for each pump/signal frequency couple. Here, the converted light frequency is centered on νc = 476.11 THz (λc = 629.7 nm). This simulation has been conducted for a 4 cm long PPLN waveguide. The poling period was Λ = 10.85 μm. In these conditions, the frequency spectral acceptance is equal to 38 GHz (wavelength spectral acceptance equal to 0.3 nm at λs = 1542.5 nm). This conversion efficiency map is directly related to the waveguide mode dispersion, mainly determined by the Lithium Niobate index dispersion and the PPLN poling period.
The red curve represents the maximum conversion efficiency, where the three interacting waves are perfectly phase matched (Δk = 0). In the pump and signal frequency ranges used here, this curve can be fitted by a linear function:
Then, the energy conservation law writes
Let us consider now an input signal with a large spectral width Δνs. We assume that the pump spectrum is flat and sufficiently broad to convert each frequency of the input signal with a maximum conversion efficiency. The spectral width of the converted field is then given by
The converted spectrum undergoes a spectral compression by a factor
The numerically estimated spectral compression factor ρnum depends on the slope bnum which is directly linked to the chromatic dispersion properties of the Lithium Niobate waveguide through Eq. (1). Here, a linear regression gives bnum = −1.213, leading to a compression factor ρnum = 4.70. Note that here bnum is negative, which means that the converted spectrum is also frequency–flipped.
The use of a broad pump spectrum is mandatory to obtain the frequency spectral compression effect. Indeed, with a monochromatic pump, the slope bnum is not defined and there is no frequency compression, as it has been investigated in  and .
3. Experimental setup
To experimentally retrieve the theoretical frequency compression effect reported in the previous section, we implemented an experimental setup based on the up-conversion interferometer previously developed in our laboratory [5, 6, 11]. In the current setup, the pump is composed of a spectral doublet to convert one input signal composed of two different frequency lines.
This interferometric configuration has two main goals. The first one is to experimentally demonstrate the frequency spectral compression . Indeed, the line frequency separation change between the input and converted doublets can be extracted from the analysis of the fringe pattern envelopes . The second goal is to validate that such a compression effect through the SFG preserves the temporal coherence of the input infrared field.
The setup is based on a Mach-Zehnder interferometer as shown on Fig. 2. The infrared spectral doublet under study is composed of a set of two balanced distributed-feedback (DFB) lasers lines at λs1 = 1542.12 nm (νs1 = 194.40 THz) and λs2 = 1543.81 nm (νs2 = 194.19 THz). The pump spectral doublet is also composed of two balanced DFB lasers lines with an emission at λp1 = 1063.3 nm (νp1 = 281.95 THz) and λp2 = 1064.3 nm (νp2 = 281.68 THz).
The infrared input signal and the pump source are both equally shared between the two interferometric arms by two polarization maintaining and single-mode fiber couplers at their operating wavelength. A 10-cm stroke fibered delay line is inserted in the arm #2 to adjust the optical path difference (OPD) between the two arms in the infrared stage of the interferometer . An optical path modulator (OPM) , with a 100-μm stroke, is inserted in the same arm for a fine OPD adjustment and to induce a temporal optical path modulation on the input signal. This allows to display the fringe pattern as a function of time. This device is driven by a triangular high voltage to induce sequenced linear OPDs as a function of time.
The infrared signal and pump sources are spectrally multiplexed together thanks to polarization maintaining fibered wavelength division multiplexers (WDM) before SFG stages on each arm of the interferometer.
In each arm, the emerging multiplexed beams are focused by achromatic injection systems in a 4 cm long Ti-indiffused PPLN waveguide with a spectral acceptance of 0.3 nm (38 GHz) around 1542.5 nm. The two non linear crystals are placed in thermally regulated enclosures to ensure a proper phase matching and achieve exactly the same expected SFG process on each arm of the interferometer. In this configuration, each line of the pump spectral doublet addresses a single line of the infrared spectral doublet to convert. We plotted the two experimental signal/pump couples on the normalized PPLN conversion efficiency curve described in the previous section (Fig. 3).
The emerging up-converted signals around λc = 629.7 nm (νc = 476.11 THz) are spectrally selected on each arm by a spectral filtering stage, composed of a dispersive prisms (P) and an interference filter (IF) centered on the mean converted wavelength. After this stage, the converted signals are spatially filtered thanks to single-mode polarization maintaining fibers at the converted wavelength. We have inserted another fiber delay line on arm #2, with a 10-cm stroke to control the OPD between the two arms of the interferometer on the visible stage. At the output of the up-conversion interferometer, we combine the two visible optical fields thanks to a polarization maintaining and single-mode fiber coupler. The resulting fringe pattern is then detected by a Silicon photodiode.
4. Demonstration of the frequency spectral compression effect
4.1. Phase propagation through the up-conversion interferometer
In this section, we explain how we can infer the frequency spectral compression effect thanks to the contrast evolution measurements as a function of the OPD applied before or after the SFG process. We focus our study on the phase propagation term of the two waves at frequencies νs1 and νs2, assuming the same intensity I0 through the two arms of the up-conversion interferometer. The equations below show the theoretical expressions of the phase terms as a function of the optical path on each stage and each arm (δIR1, δIR2, δVi1, δVi2) for the input and output spectral doublet (νs1, νs2 and νc1, νc2).
|Infrared stage (νs)||Visible stage (νc)|
|Phase difference (Δφ=φ2 − φ1)|
The OPM temporally modulates the phase of the input infrared signal on arm #2 and the optical path on this arm can be written as . Thanks to the property of phase conservation in a sum frequency generation process , the phase differences on the infrared stage (Δφs1, Δφs2) are transferred to the input of the visible stage on each arm of the interferometer. Then, the fringe pattern at the output of the interferometer can be written for each frequency νs1 and νs2 as a function of the optical path variation
The contrast evolution of the fringe pattern is driven by the two different beat lengths related to the signal wave before and after the SFG process. When applying an OPD between the two interferometric arms on the infrared stage, the envelope period is equal to the beat length LbIR. Conversely, when applying an OPD on the visible stage, the envelope period is equal to the beat length LbVi. Due to the phase matching condition in the experimental configuration, the frequency spectral separations Δνs and Δνc are different before and after, the SFG process, and therefore the beat lengths are different between the infrared and the converted signals. Consequently, an OPD applied before and after SFG will not have the same impact on the fringe visibility, demonstrating a frequency spectral compression effect.
4.2. Experimental results
In a first step, we measured the maximum fringe contrast at the up-conversion interferometer output for a single line infrared signal (νs1) converted thanks to a single line pump source (νp2). The aim of this measurement was to calibrate the reliability of the setup. In this configuration, we obtained a contrast equal to 98.2%. When operating simultaneously with the two frequency pairs (νs1 − νp2 and νs2 − νp1), the maximum contrast reached 98%. The contrast loss can be explained by a polarization control defect over the visible stage of the interferometer [14, 15]. However, these results prove our ability to implement several SFG processes simultaneously in the up-conversion interferometer arms while preserving a high contrast level.
To demonstrate the frequency spectral compression effect, we conducted measurements of the fringe contrast evolution for a balanced infrared spectral doublet. We measured the fringe contrast evolution as a function of the OPD between the two arms of the interferometer by applying this OPD either on the infrared stage or on the visible stage.
Figure 4 shows the experimental fringe contrast versus the OPD applied on the infrared stage. The beat length of the fringe pattern envelope is equal to LbIR = 1.46 mm. When the OPD is applied on the visible stage (Fig. 5), the beat length LbVi is equal to 5.97 mm. The periodicity difference between these contrast curves experimentally demonstrates a frequency spectral compression effect resulting from a multipump configuration in a sum frequency generation process. The experimental frequency spectral compression factor ρexp is equal to
We note here a relative difference (≈ 12.8%) with the simulated frequency compression factor ρnum = 4.70. This difference is mainly due to the fact that each frequency couple νs1 − νp2 and νs2 − νp1 is not perfectly located on the maximum conversion efficiency curve (Fig. 3) because of the wavelength resolution limit (≈ 50 pm corresponding to ≈ 6.3 GHz at 1542.5 nm) on the DFB sources. The experimental slope bexp, deduced from the signal/pump couple positions on Fig. 3, is equal to −1.245 instead of the previously numerically obtained slope bnum = −1.213 (relative difference of 2.6% between the two slopes). As the frequency spectral compression factor depends on this conversion efficiency slope, we retrieve the experimental conditions
5. Demonstration of the temporal coherence information conservation
To experimentally verify that the converted field carries the same temporal coherence information than the input infrared field, we measured the contrast evolution as a function of an OPD applied on the infrared stage in one hand and on the visible stage in the other hand. To obtain a clearly identifiable temporal coherence signature, we strongly unbalanced the input signal doublet. The relation between the intensity of each line at and is now , with α the unbalance ratio between the two line intensities.
5.1. Experimental results
We unbalanced the infrared spectral doublet at the interferometer input by a factor α = 13.5. In a first step, we measured the contrast evolution at the interferometer output as a function of an OPD applied on the infrared stage of the setup (Fig. 6). The constrast modulation amplitude is equal to 6.24%. The red curve represents the theoretical visibility function V (δ) of the fringe pattern contrast evolution calculated for the experimental conditions, using the Wiener-Khintchine theorem 
In a second step, we conducted a measurement of the contrast evolution as a function of an OPD applied on the visible stage (Fig. 7).
The contrast modulation amplitude is equal to 6.09%, to be compared with the 6.24% modulation amplitude obtain when applying the OPD on the infrared stage. These results are in very good agreement (relative difference of 2.4%). We retrieve the same modulation amplitude before and after the SFG process, that clearly demonstrates the conservation of the temporal coherence information of the signal through a SFG process powered by a pump spectral doublet.
Finally, the beat length measured when the OPD is applied on the infrared stage (resp. on the visible stage) is LbIR = 1.43 mm (resp. LbVi = 5.79 mm), leading to a frequency spectral compression factor ρexp = LbVi/LbIR = 4.05. This result is in good agreement with the experimental value ρexp = 4.09 obtained in section 4 (relative difference between the two measurements lower than 1.5%) and demonstrates the reliability of the experimental setup.
In this paper, we have numerically and experimentally demonstrated the frequency spectral compression effect of an infrared signal through a SFG process powered by a pump spectral doublet. We obtained an experimental compression factor ρexp = 4.09 with a high reliability.
We also verified that this compression effect does not deteriorate temporal coherence information of the infrared input signal under analyze and will permit to relax the constraints on the optical path equalization in a fibered interferometer, allowing an easier implementation while analyzing a broadband source.
Moreover, analyzing a broadband infrared source through an up-conversion interferometer, powered by a pump source in multipump configuration, will permit to benefit from a maximum frequency compression effect , increasing the coherence length of the source, while preserving its temporal coherence information.
This work has been financially supported by the Centre National d’Études Spatiales (CNES) and by the Institut National des Sciences de l’Univers (INSU). Our thanks go to A. Dexet for the development and his advices for all the specific mechanical components.
References and links
2. A. P. VanDevender and P. G. Kwiat, “High efficiency single photon detection via frequency upconversion,” J. Mod. Opt. 51, 1433–1452 (2004).
4. K.-D. Bchter, H. Herrmann, C. Langrock, M.M. Fejer, and W. Sohler, “All-optical Ti:PPLN wavelength conversion modules for free-space optical transmission links in the mid-infrared,” Opt. Express 34, 470–472 (2009).
5. S. Brustlein, L. Del Rio, A. Tonello, L. Delage, F. Reynaud, H. Herrmann, and W. Sohler, “Laboratory demonstration of an infrared-to-visible up-conversion interferometer for spatial coherence analysis,” Phys. Rev. Lett. 100, 153903 (2008). [CrossRef] [PubMed]
6. D. Ceus, A. Tonello, L. Grossard, L. Delage, F. Reynaud, H. Herrmann, and W. Sohler, “Phase closure retrieval in an infrared-to-visible upconversion interferometer for high resolution astronomical imaging,” Opt. Express 19, 8616–8624 (2011). [CrossRef] [PubMed]
7. R. T. Thew, H. Zbinden, and N. Gisin, “Tunable upconversion photon detector,” Appl. Phys. Lett. 93, 071104 (2008). [CrossRef]
9. S. Wabnitz, A. Picozzi, A. Tonello, D. Modotto, and G. Millot, “Control of signal coherence in parametric frequency mixing with incoherent pumps: narrowband mid-infrared light generation by downconversion of broadband amplified spontaneous emission source at 1550 nm,” J. Opt. Soc. Am. B 29, 3128–3135, (2012). [CrossRef]
10. R. W. Boyd, Nonlinear Optics (Academic Press, New York, 2008), pp. 69–96. [CrossRef]
11. L. Del Rio, M. Ribiere, L. Delage, and F. Reynaud, “First demonstration of a temporal coherence analysis through a parametric interferometer,” Opt. Commun. 281, 2722–2726 (2008). [CrossRef]
12. L. M. Simohamed, L. Delage, and F. Reynaud, “An optical delay line with a 318 mm stroke,” Pure Appl. Opt. 5, 1005–1009 (1996). [CrossRef]
13. L. Delage, F. Reynaud, and A. Lannes, “A laboratory imaging stellar interferometer with fiber links,” Appl. Opt. 39, 6406–6420 (2000). [CrossRef]
14. G. Huss, L. M. Simohamed, and F. Reynaud, “An all guided two-beam stellar interferometer: preliminary experiment,” Opt. Commun. 182, 71–82 (2000). [CrossRef]
15. G. Huss, F. Reynaud, and L. Delage, “An all guided three-arm interferometer for stellar interferometry,” Opt. Commun. 196, 55–62 (2001). [CrossRef]
16. M. Born and E. Wolf, Principle of Optics (Pergamon Press, London, 1964), pp. 503–504.