## 1 Introduction  
The core task of quantum communication is to realize remote transmission of quantum information [1, 2]. However, direct transmission of quantum information over long distance (\(>\)1000 km) in optical fiber network is intractable because of exponential loss of the transmitted photons. To solve this problem, the concept of quantum repeater is proposed, in which quantum storage plays a key role [3, 4, 5]. Atomic ensemble [6, 7], single atom [8, 9], trapped ions [10, 11] and solid-state systems [12, 13] are all potential candidates for quantum repeater, in which quantum storage has been actively performed. However, the photons emitted from these systems are in the visible to near infrared range, where the loss in optical fiber is significant. Therefore, it is a basic requirement of long-distance quantum communication to transform the emitted photons into communication band [14, 15, 16, 17]. The frequency conversion from storage band to communication band also provides the foundation for the construction of heterogeneous quantum network, in which network nodes are served by different storage mediums. One can take advantage of various capabilities offered by diversity of quantum nodes [18, 19]. The difference frequency generation (DFG) in nonlinear crystal is often used for frequency down conversion [20, 21, 22, 23, 24]. In recent years, it is a very attractive research field to convert light field from storage band to communication band by using DFG. The combination of emissive quantum memory and frequency conversion with DFG is the most promising scheme for building quantum repeater [25; 26; 27; 28]. The frequency conversion to communication band has been demonstrated from different wavelengths resonant with various storage mediums, including atoms [15; 29], ions [30; 31], quantum dot [16], nitrogen-vacancy centers [32; 33], etc. Rubidium (Rb) atom is usually used for quantum storage. Great progress has been made in the field of photon-atom entanglement quantum interface based on Rb atomic ensemble [6; 14; 34; 35; 36; 37; 38]. Most of them work at 795 nm which corresponds to the transition of \({}^{87}\)Rb D1 line, because the transition line has a concise energy level structure which can realize clean and effective manipulation of atomic internal states. However, there are few experimental reports on frequency conversion from 795 nm to communication band [14; 28]. Using four-wave mixing in cold atoms with large optical thickness, [PERSON] et al., achieved the conversion of 795 nm to the communication O-band 1367 nm [14], however, the target wavelength of the conversion is strictly dependent on the atomic energy level. [PERSON] et al., converted the 795 nm light to the O-band 1342 nm by using DFG in periodically poled lithium niobate (PPLN) waveguide [28]. So far, there is no experimental report on the conversion from 795 nm to C-band or L-band. By using pumping light at 1620 nm, the 795 nm photons can be converted to C-band 1561 nm. However, the commercial amplifier at 1620 nm is expensive, and its output power is relatively low. In this paper, we report the first experimental study to convert light field from 795 nm to L-band 1621 nm by using DFG in PPLN waveguide. The transmission loss of L-band in fiber channel (0.18 dB/km) is lower than that of O-band (0.3 dB/km), which will significantly improve the successful probability of long-distance entanglement distribution [4; 39]. L-band is also the mainstream choice for expanding communication bandwidth at present due to low transmission loss [40; 41]. In DFG process, the strong pumping beam causes spontaneous Raman scattering (SRS) noise, which typically covers over a region of several hundreds of nanometers. The noise suppression is important for improving the signal-noise ratio (SNR) of the converted field, especially for the frequency conversion of quantum state light field [15; 22; 42; 43; 44].