Fiber-based narrowband bright squeezed light generation by double-pass parametric amplification
Tianyi Tan, Changsheng Yang, Qilai Zhao, Chengzi Huang, Xianchao Guan, Zhongmin Yang, Shanhui Xu
11 Fiber-based narrowband bright squeezed light generation bydouble-pass parametric amplification T IANYI T AN , C HANGSHENG Y ANG , Q
ILAI Z HAO , C HENGZI H UANG ,X IANCHAO G UAN , Z
HONGMIN Y ANG , AND S
HANHUI X U State Key Laboratory of Luminescent Materials and Devices and Institute of Optical CommunicationMaterials, South China University of Technology, Guangzhou 510640, China Guangdong Engineering Technology Research and Development Center of Special Optical FiberMaterials and Devices, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South ChinaUniversity of Technology, Guangzhou 510640, China School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510640,China Guangdong Engineering Technology Research and Development Center of High-performance FiberLaser Techniques and Equipments, Zhuhai 519031, China [email protected] [email protected] The squeezed states of light become more and more important in the fields of quantumenhanced precision measurement and quantum information. To get this vital continuousvariable quantum resource, the generation of squeezed states of light becomes a key factor. Inthis paper, a compact telecom fiber-based bright squeezed light (BSL) generator isdemonstrated. To our knowledge, this is the first time that BSL has been reported in a fiber-based system to date. To obtain the BSL, a double-pass parametric amplifier based on surface-coated lithium niobate waveguide is employed. When the 1550 nm seed laser of theparametric amplifier is blocked, a stable 1.85 dB squeezed vacuum is obtained. With injectedseed power of 80 μW, an output power of 18 μ W and a squeezing value of 1.04 dB areachieved of the BSL at 1550 nm. Due to the good mode matching in the fiber and the absenceof the resonant cavity, this flexible and compact BSL generator has the potential to be usefulin out-of-the-laboratory quantum technologies. Moreover, the BSL has a narrow spectralwidth of 30 kHz, which is inherited from a narrow-linewidth single-frequency seed laser. Inaddition to being free from the wavelength-dependent losses, the narrowband BSL is alsobeneficial to improve the signal-to-noise ratio of quantum-enhanced precision measurement.
1. INTRODUCTION
The generation of squeezed states of light has been an important technology to preparecontinuous variable quantum resources, which have been the foundations of variousapplications, such as quantum information [1, 2] and quantum computing [3]. In addition totheir potential in information technology, squeezed states of light are also active in systemstrapped by quantum noise limit to achieve the ultimate accuracy, such as quantum imaging [4,5], quantum enhanced precision measurement [6-8], and especially gravitational wavedetection [9].In recent years, with the gradually mature development of the squeezing technologies basedon the bulk-optics systems, advanced squeezing sources have been reported at various optionalwavelengths towards different applications. To date, thanks to the small transmission loss in thefree-space, the cavity-enhanced technology and the strict mode matching, the squeezingvacuum sources with the highest squeezing value of 15, 13, and 4 dB are achieved at 1.0, 1.5,and 2.0-μm band, respectively [10-12]. However, the sophisticated structure and huge scale ofthe squeezing systems limit the popularization of this valuable technology in scientific andtechnical researches. In response to this difficulty, a series of miniaturized squeezing sourcesbased on fiber or on-chip systems have been implemented [13-16]. Miniaturization of the semi-monolithic optical parametric oscillator is one of the viable methods, by which a 6.2 dBsqueezing source has been realized [17, 18]. For further simplification of the squeezing source,the single-pass spontaneous parametric down conversion (SPDC) has become a decent choicefor generating the squeezed vacuum. So far, the highest squeezing value achieved in fiber-basedsystem by single-pass SPDC is 3.2 dB [19]. The development of these fiber-based generators ofsqueezed vacuum has great benefits in improving the accuracy of precision measurements,especially in fiber interferometers [20-22].The difference from the squeezed vacuum state is that the bright squeezed light (BSL) has acertain coherent amplitude, thus it can be directly used as a signal in some quantum-noise-limited systems, such as quantum imaging [23] and optomechanical magnetometry [24]. On theother hand, the BSL can also be used as the continuous variable quantum entanglementresource in applications such as quantum dense coding and quantum teleportation [25-27].Generally, without the restriction of the resonator, the squeezed vacuum generated by a single- pass SPDC has a broadband spectrum (tens of nanometers), which is determined by the phasematching bandwidth of the non-linear medium [28]. However, the broadband characteristicmeans that the applications of such squeezing sources in the systems with wavelength-dependent loss will be limited, such as wavelength division multiplexing systems. In contrast,due to the narrowband spectrum, which is mainly determined by the seed linewidth in opticalparametric amplification (OPA), the BSL can solve such problems even without the interventionof the resonant cavity. There are already excellent results of the generation of BSL in bulk-opticssystems [29-31]. However, to our best knowledge, the narrowband BSL generated in fiber-based system has been less investigated to date.In this paper, we report a telecom fiber-based narrowband BSL generated by a double-passparametric amplification which has a squeezing value of 1.04 dB and a spectral bandwidth of 30kHz. The whole system employs fiber-based optical components requiring no alignmentprocedures for spatial mode matching. These advantages guarantee extreme reliability andmake our approach a valuable candidate for real-world applications based on BSL.
2. EXPERIMENTAL SETUP
The experimental setup is shown in Fig. 1. A 1550 nm single-frequency distributed feedbacklaser diode (DFB-LD) is employed as the laser source of the system. It has an intensity noiseequal to the quantum noise limit in the frequency range greater than 200 kHz and a linewidth ofless than 30 kHz. The DFB-LD with 50 mW output is firstly divided by two 50:50 opticalcouplers (OCs) into local oscillator (LO), seed coherent state, and fundamental laser.The fundamental laser with a power of ~20 mW is then amplified to 800 mW by a master-oscillator power amplifier (MOPA) with a 3.5-m-long Er /Yb -codoped double - cladding fiber.The boosted fundamental laser is then injected into the periodically-poled LiNbO waveguide 1(PPLN WG1) for the second harmonic generation (SHG). At a phase-matching temperature of37.3 ℃ , the SH laser with a power of 110 mW, which is used as the pump laser of the OPA, isoutput and transmitted through the polarization-maintaining (PM) single-mode fiber (Nufern780HP). In order to avoid the influence of residual pump on squeezing value of the BSL, theresidual fundamental laser is filtered out by a 4-cm-diameter fiber loop with a segment of780HP fiber, which does not support the fundamental mode of 1550 nm laser (not represented in Fig. 1) [32]. In addition, the power of the pump injected into PPLN WG2 is controlled by thevariable optical attenuator 3 (VOA3) during the experiment. Fig. 1.
The experimental setup of the narrowband BSL generation. A 1550 nm single-frequency low-noisedistributed feedback laser diode (DFB-LD) is divided into local oscillator (LO), seed coherent state (SEED), andfundamental laser by two 50:50 fiber optical coupler (OC). The fundamental laser is boosted by a master-oscillator power amplifier (MOPA) and subsequently injected into the periodically-poled LiNbO waveguide 1(PPLN WG 1). The generated 775 nm laser by second harmonic generation (SHG) is then guided into the PPLNWG 2 as the pump. The SEED is phase controlled by a piezoelectric fiber stretcher (PS2) and then backwardguided into the PPLN WG 2 by a 1:99 OC. The seed coherent state interacts with the pump to generate thebright squeezed light (BSL) in the double-pass optical parametric amplification (OPA). The power, polarizationand phase of LO are manipulated by a variable optical attenuator (VOA1), a polarization controller (PC) andPS1. Eventually, the BSL signal interferes with the LO on a homemade exquisite balanced OC. And the noiseperformance is monitor by a balanced homodyne detector (BHD). The schematic of the double-pass parametric amplification is shown in Fig. 2. The input andoutput facets of the PPLN WG2 are coated with high-reflective (HR) film corresponding to theseed (1550 nm) and the pump wavelength (775 nm), respectively. And the reflectivity of the HRcoating at both facets is greater than 99.99%. The pump laser from SHG is forward guided intothe WG2 and reflected back at the output facet. Similarly, the quantum-noise-limited seedcoherent state is backward guided into the WG2 by a 1:99 OC and reflected back at the inputfacet. Meantime, the OPA occurs during the double-pass propagation of the pump and seed laserin WG2. The gain factor of amplification or deamplification depends on the relative phasebetween two beams which is manipulated by a piezoelectric fiber stretcher (PS). The power ofthe seed coherent state finally guided into the WG2 is ~80 μW, which is controlled by the VOA2,and the final output power of the BSL signal is about 18 μW.
This double-pass parametric amplification structure doubles the effective nonlinearinteractional length in the WG2. In addition, it has other significance. Firstly, it is solved the issuethat injection of the seed coherent state from the same fiber as the pump is impossible due tothe nonsupport of 1550 nm fundamental mode in the 780HP fiber. Secondly, the HR coating of1550 nm prevents noisy photons coupling into the parametric amplifier, which may reduce thesqueezing factor of the BSL. Finally, the double pass of the pump means that it does not passthrough the parametric amplifier, which implies that it is not necessary to insert an additionalfiber filter with superfluous insertion loss.Moreover, to avoid severe phase jitter caused by environmental noise, the entire systemexcept the LO path is passively isolated from sound and vibration. Furthermore, thetransmission fibers of the seed, the pump and the BSL are all PM single-mode fiber whichinsensitive to the environmental noise.
Fig. 2.
Schematic of the double-pass OPA. The enlarged model of the PPLN WG2 and the propagation paths ofthe775 nm pump laser (red) and the 1550 nm seed coherent state (green) in WG2 are shown in this figure. Theinput and output facets of the WG2 are coated with high-reflective (HR) film corresponding to 1550 nm and775 nm, respectively.
The noise performance of the generated BSL is detected by a balanced homodyne detector(BHD). The power of LO is adjusted to 5 mW by VOA1, which is 278 times as powerful as that ofthe BSL. The phase of the LO is scanned by a piezoelectric fiber stretcher (PS) under a 5 Hztriangle-wave modulation. To ensure enough interference efficiency, a polarization controller(PC) is used to adjust the polarization state of the LO to maintain the same of that as the BSLsignal. In our experiments, the measured efficiency of interference is greater than 97%. The BSLsignal interferes with the LO on a homemade exquisite balanced OC, which has a insertion lossof 0.21 dB and a ratio deviation of less than ±2%. Moreover, the two output arms of thebalanced OC are adjusted to the same length. To ensure the squeezing value of the BSL, a pair of photodiodes with the quantum efficiency of greater than 99% is employed in the BHD whichincludes built-in filters, amplification and differential circuits. The total system transmissionefficiency is estimated to be 0.533, which includes waveguide transmission efficiency of 0.75,the fiber transmission efficiency of 0.74, interference efficiency of 0.97, and detection efficiencyof 0.99.
3. RESULTS AND DISCUSSION
The normalized measured parametric gain of the OPA is shown in Fig. 3. When the pump laseris absent, an output power of ~29 μW is obtained with ~80 μw seed injection due to the loss.Subsequently, considering whole coupling and material losses, with a pump power of 60 mWand a seed laser power of ~80 μW guided in the OPA, the maximum and minimum outputpower is ~147 μW and ~18 μW, respectively. That means, when the phase of the seed coherentstate relative to the pump is varied, the output power is amplified up to 5.06 times anddeamplified down to 0.62 times. In order for OPA to work in the deamplification state togenerate quadrature amplitude BSL, it is necessary to control the phase shifter to minimize theDC component of the output of the photodetector [33].
Fig. 3.
Normalized output power of the OPA versus phase scanning over time with a pump power of 60 mW .The maximum gain of amplification is 5.06 times, and the minimum gain of deamplification is 0.62 times.
The squeezing curves of generated squeezed vacuum and BSL are shown in Fig. 4. (a) and (b),respectively, which are measured by scanning the phase of the LO. When the seed light isblocked by VOA2, a squeezed vacuum with 1.85 dB squeezing and 2.7 dB anti-squeezing isobtained with 60 mW pump. At the same pump power, 80 μw of seed coherent state is injectedinto the OPA, meanwhile, the PS2 is adjusted to minimize the DC component of the BHD output, which means that the OPA works in the deamplification state. Finally, about 20 μW BSL isobtained with 1.04 dB squeezing and 2.0 dB anti-squeezing. Obviously, when there is no phaselocking between the LO and the seed laser, even if passive isolation from environmental noise istaken, the squeezing value of BSL still be affected by the phase jitter. For further development,relative phase locking technology can improve and stabilize this type of generation of BSL.
Fig .4.
Normalized noise power (red) at 2.5 MHz as a function of the phase scanning time (with 3 Hz triangle-wave modulation). The analyzer resolution and the video bandwidths of the electronic spectrum analyzer are100 kHz to 10 Hz, respectively. The quantum noise limit (blue) is measured with average when squeezing isblocked. (a) Squeezed vacuum without seed injection. (b) BSL with seed coherent state injection.
The optical spectrum of the generated squeezed vacuum and BSL are shown in Fig. 5. (a) and(b). Due to the strength of the squeezing vacuum is too weak to be measured directly with theoptical spectrum analyzer (OSA), an indirect measurement method using photon counting isused by employing a tunable fiber filter with a 140 nm tunable width and an avalanchephotodiode based single photon counter. With the 5 nm tuning step and the 1 nm filterbandwidth, the estimated spectral width is about 60 nm based on the curve in Fig.5 (a), which isdetermined by a phase match acceptance bandwidth of the nonlinear process. The spectrum of the BSL directly measured by an OSA is shown in Fig. 5. (b), it is exactly the same as that of theseed laser. With the self-heterodyne method involving a 48.8 km fiber delayed Mach-Zehnderinterferometer, the linewidth of the 1550 nm seed laser is measured to be 30 kHz. It can beconservatively speculated that the linewidth of the BSL produced by the double-pass OPA isapproximately equal to 30 kHz. This means that, unlike broadband squeezing vacuum, thenarrowband characteristic of the BSL make it unrestricted in wavelength-dependent losssystems. Further, the narrowband characteristic will make it perform better in the field ofprecise measurement such as fiber sensing systems.
Fig. 5. (a) Spectrum of the squeezed vacuum estimated by counting the number of photons at differentwavelengths. (b) Spectrum of the BSL measured by an optical spectrum analyzer with a resolution of 0.016nm.
In conclusion, a telecom fiber-based narrowband BSL generated by double-pass parametricamplification is developed. A squeezing value of 1.04 dB and a spectral bandwidth of 30 kHz areobtained. As far as we know, this is the first time that BSL has been reported in a fiber-basedsystem to date. Although this BSL is still subject to large loss and phase jitter of the system now,with the advancement of materials and device technology, and the introduction of phase lockingtechnology, this kind of BSL based on double-pass parametric amplification will likely be widelyused in the field of quantum precision measurement and quantum information with its simpleand fiber-based characteristics [34, 35].
Funding Information.
National Key Research and Development Program of China(2017YFF0104602), Major Program of the National Natural Science Foundation of China(61790582), NSFC (61635004, 11674103, 61535014 and 51772101), Guangdong KeyResearch and Development Program (2018B090904001 and 2018B090904003), LocalInnovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137), Guangdong Natural Science Foundation (2017A030310007), the Science andTechnology Project of Guangdong (2016B090925004 and 2017B090911005), and the Scienceand Technology Project of Guangzhou (201804020028).
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