I.

Introduction

Nowadays, several couples IR laser/hyperfine iodine lines have already been used to build up optical frequency standards, with demonstrated stabilities in the 10^-15 range: Frequency doubled Nd: YAG/I₂ (532 nm) [1-3], frequency doubled Yb doped fiber laser/I₂ (515 nm) [4], frequency tripled LD/I₂ (514 nm) [5]. The well-known modulation transfer spectroscopy (MTS) technique is used in all mentioned works to detect very narrow Doppler-free hyperfine iodine lines. The emergence of high efficient non-linear waveguide crystals, combined with the existence of infrared (IR) laser sources with high technological maturity, issued from Telecom development, offer real opportunities to implement reliable and very compact stabilized laser devices of great relevance for space applications. Our proposed frequency tripling process open the way to link in easy way very performant Telecom laser sources in terms of compactness, high level of emitted optical power and impressive intrinsic phase noise, to high quality factor Doppler-free saturated iodine hyperfine lines existing in the green range.

In this paper, we describe a new approach combining the use of frequency tripled Telecom laser diode (TLD) operating in the 1.5 µm range and one from thousands of narrow iodine hyperfine lines existing in the green part of the visible spectrum for the frequency stabilization purpose. This all fibered system emits simultaneously three cw-coherent and stabilized radiations. Compared to other IR laser sources, the TLD exhibit unprecedented intrinsic phase noise (linewidth < kHz), before any electronic feedback, associated to an extremely small volume (~ few cm³) and optically fibered mode operation. Furthermore, iodine lines around 515 nm have remarkable quality factor (Q > 2 × 10⁹) [6], achievable with simple and compact experimental interrogation configurations using the well-known MTS technique.

II.

Frequency Tripling Process

Third harmonic generation (THG) of continuous wave (CW) infrared lasers has been demonstrated in only few cases, with very poor efficiency P3ω/Pω [7, 8]. In early 2002, a first attempt to observe iodine lines via a THG of a telecom laser has been described using two second order nonlinear processes in a unique crystal [9]. The two processes were operated in a single periodically poled Lithium Niobate crystal (PPLN) allowing a green power generation at level of few tens of nW. The associated optical conversion from IR to green η = P3ω/Pω is in the range of 10^-5 %. The main limitation is due to the difficulty to fulfill the quasi phase matching conditions for second harmonic generation (SHG) and sum frequency generation (SFG) simultaneously in the same nonlinear crystal. Recently, higher efficiency has been demonstrated at level of η = P3ω/Pω = 0.25 %, corresponding to 1.5 mW generated green light, with two cascaded crystals used to fulfill two second order steps: (ω + ω → 2ω) followed by (ω + 2ω → 3ω) [10].

Following this demonstration, we propose a new optical architecture using two fibered ridge waveguide PPLN nonlinear crystals [11]. We utilize two optically fibered ridge waveguide Zn doped PPLN crystals to achieve a SHG process followed by a SFG process in an original optical arrangement as depicted in Fig. 1. We generate up to P3w = 300 mW at 514 nm using Pω = 200 mW associated to red power P2ω = 330 mW achieved independently from PPLN1 with 600 mW at w. Consequently, this maximum output green power was obtained from 0.8 W total IR power at 1.542 µm, corresponding to an optical conversion efficiency η = P3ω/Pω ~ 36 %. During our measurements, synthesized in Fig. 3, the total optical power incident onto the SFG crystal was intentionally limited to ~ 0.55 W (Pω + P2ω) in order to avoid possible optical damage.

Fig. 1:

Principle of the THG setup: (ω + ω → 2ω) + (ω + 2ω → 3ω). A= Erbium Doped Optical Fibre Amplifier, PPLN1 = Optical fibered PPLN crystal for SHG operation, PPLN2=optical fibered PPLN crystal for SFG operation, DM: Dichroic mirror.

The IR laser source used in this work is a butterfly type narrow linewidth laser diode (linewidth ~ 1 kHz, power ~10 mW) followed by an erbium doped optical amplifier (EDFA) delivering up to 1 W over the full C band of the telecom range. All optical fibers, splitters and optical isolators used in this setup are polarization maintaining devices (Fig. 2).

Fig. 2:

Frequency tripled Telecom laser diode

Fig. 3:

Evolution of the measured third harmonic power Versus input optical powers at 1542 nm and 771 nm.

III.

Frequency Stabilization Setup

We use the well-known saturated absorption approach associated to the MTS technique to frequency stabilize the 1.542 µm laser diode against an iodine hyperfine line in the green (Fig. 4). We have used the a₁ hyperfine component of the R 35 (44-0) ¹²⁷ I₂ line [12] at ~ 514.017 nm for a preliminary frequency stabilization evaluation. The pump beam (respectively probe beam) is frequency shifted by 79 MHz (resp. 80 MHz) with acousto-optic modulators. A low frequency modulation (220 kHz) is applied to the pump beam thanks to an electro-optic modulator to detect the atomic saturation signal. The two optical laser beams of diameter ~2 mm are carefully collimated and overlapped in the 20 cm long iodine cell. The interaction length is extended up to 1.2 m thanks to 6 successive optical passes in the cell. Both internal and external faces of the cell windows are antireflection coated in the green [13]. The quartz cell is filled with highly pure iodine in Institute of Scientific Instruments in Czech Republic.

Fig. 4:

Iodine stabilization optical setup. BS: beam splitter, M: Mirror, AOM: Acousto-optic modulator, EOM: electro-optic modulator, PBS: polarizing beam splitter.

The saturated absorption signal is detected by a balanced silicon photodiode. A part of the probe beam is split off before propagating in the cell in order to eliminate common noise of the laser probe beam. The probe and pump powers are stabilized with signals detected with two independent photodiodes. An additional photodiode (not shown in fig. 4) is used for a permanent control of the residual amplitude modulation (RAM) associated to the phase modulation of the pump beam. The cold finger temperature of the cell is regulated around -15 °C within 1.5 mK, using an homemade electronic PID controller. The corresponding vapor pressure in the cell is estimated about 1 Pa.

An independent stable frequency reference laser (FRL) is utilized to fulfill the frequency stability measurement of our 1.5 µm iodine optical frequency standard. The FRL is based on another IR laser source frequency locked to an ultra-stable optical cavity described elsewhere [14]. It is located in a separate building and is connected to our experiment by a 200 meters long fiber link. During this preliminary measurement the frequency noise of this optical link was not compensated, because its contribution together with the reference cavity instability exhibit an Allan deviation at level of ~10^-15 over the full integration time measurement. Subsequently, the frequency stability evaluation of our iodine stabilized laser is not affected.

Figure 5 reports the preliminary short term frequency stability measurement of the iodine stabilized laser diode. It shows an Allan deviation decreasing with a slope of 4.8 × 10^-14 τ^-1/2 with a minimum value of 6 × 10^-15 for 50 s of integration time. During these preliminary measurements the residual amplitude modulation (RAM) was not compensated and could explain the behavior of the frequency stability observed for τ > 50 s. The Allan deviation shown in Fig. 5 is associated to the measured frequency beat note between the iodine stabilized laser and the cavity reference laser. Hence, none frequency drift has been subtracted.

The corresponding frequency noise in terms of linear spectral density shown in Fig. 6 is compared to the LISA requirement: between 0.1 mHz and 1 Hz.

Fig. 5:

Frequency stability of the iodine stabilized 1542 nm laser diode using MTS technique.

Fig. 6:

Frequency noise of the iodine stabilized 1542 nm laser diode Red line: LISA requirement

This frequency stability reported above is achieved using the usual MTS technique, where only the pump beam is frequency modulated (Fig. 4). We are investigating the potential of a new approach based on frequency modulation technique (FM), associated to a 3^rd harmonic detection of iodine lines. Thus, we build up a preliminary optical setup based upon this concept, directly connected to our THG laser setup (Fig. 2). We start measurement with this new approach and achieve a very preliminary short term frequency stability measurement at the level of 2 × 10^-13 at 1s (Fig. 7). This new approach currently under evaluation - and improvement- could be highly relevant for space applications. A final very compact optical setup design (volume < 3 l) based on this approach is under development (Fig. 8).

Fig. 7:

Preliminary frequency stability of the iodine stabilized 1542 nm laser diode Using FM technique and 3^rd harmonic iodine line detection

Fig. 8:

New optical bench using frequency modulation technique Under development in the frame of SYRTE-SODERN collaboration

IV:

Conclusion

We have presented our ongoing developments in realizing a frequency-stabilized-compact and fibered frequency reference laser source in the optical Telecom domain. We have already demonstrated frequency stability in the 10^-15 range, using a compact tripled IR laser associated to a “conventional” free space optical bench for the iodine Doppler-free spectroscopy. This result shows a great potential for obtaining high stability laser reference for future space mission requiring together accuracy and frequency stability.

On the other hand, we investigate a new approach based on a different frequency modulation/detection technique in order to develop a more compact optical setup.