定义：
是研制成功的第一种气体激光器，也是最常用的一种，通常在可见光频段（6328Å）工作，其他还有1.1523μm及3.3913μm，但不常用。功率一般约数毫瓦，连续发光。因为制造方便、较便宜、可靠，所以使用较多。由于单色性好，相干长度可达数十米以致数百米。

介绍
      氦氖激光器是以中性原子气体氦和氖作为工作物质的气体激光器。以连续激励方式输出连续激光。在可见光和近红外区主要有0.6328微米、3.39微米和1.15微米三条谱线，其中0.6328微米的红光最常用。 氦氖激光器的输出功率一般为几毫瓦到几百毫瓦。
结构分类
氦氖激光器结构一般有三种形式：
 

     ①内腔式(结构如 图1)。放电管与谐振腔固定在一起。


     ②外腔式。放电管与谐振腔完全分开。
  ③ 半内腔(或半外腔)式。谐振腔中的一块反射镜与放电管固定在一起，另一块则与放电管分开。 

        放电毛细管内充以氦氖混合气体，其气压比为 5∶1到10∶1，总压强为133.3～266.6帕(1～2 毫米汞柱)。激光工作物质是氖，起受激辐射作用，氦为辅助气体，起传递能量作用。放电管的阳极一般用钨棒制成，阴极为铝或钼圆筒。
        内腔式激光器使用方便，无需调整谐振腔，反射镜 和放电管之间没有窗片相隔，光能损失少，但因放电使管受热变形导致激光输出下降，稳定性变差，且管长度越长，稳定性越差。
        外腔式激光器放电管两端固定两块呈一特殊倾角的窗片。 反射镜安装在窗片外，可进行调整以适应多种实验的要求。腔体受温度影响较小，输出的激光为线偏振光。缺点是使用不方便，窗片与轴线的夹角不易调准，影响输出功率。
        半外腔式激光器综合上述两种结构的优点，是一种比较理想的激光器。氦氖激光器的输出功率和激光频率的稳定性容易控制，光束的单色性、相干性和方向性均好，器件寿命长，结构简单，价格较低，在工农业生产、科研和教学等方面的应用日趋广泛，尤其在精密计量、准直导向、流体的流速流量测量和全息照相等方面发挥着重要作用。
应用

        氦氖激光器已经被人们应用得非常普遍。但氦氖激光器又存在一定的缺点，激光器的效率较低，功率也不够大。所以在激光外科手术、钻孔、切割、焊接等这些行业中，人们现在大多换成采用 CO2激光器、脉冲激光器或者是半导体激光器等大功率激光器。因为氦氖激光器具有工作性质稳定、使用寿命比较长的特点，因而现在对于氦氖激光器在流速和流量测量方面得到了更加普遍的开发和利用，同时在精密计量方面的应用也非常广泛。

Definition: gas lasers based on a helium–neon mixture

More general term: gas lasers

Helium–neon (He–Ne) lasers are a frequently used type of continuously operating gas lasers, which is also the first demonstrated gas laser (already in 1961 [1]).

Most often, He–Ne lasers emit red light at 632.8 nm at a power level of a few milliwatts and with excellent beam quality. The gain medium is a mixture of mostly helium and some neon gas in a glass tube, which normally has a length of the order of 15–50 cm.

Figure 1: Setup of a helium–neon laser.

A DC current, which is applied via two electrodes with a voltage of the order of 1 kV (but higher during ignition), maintains an electric glow discharge with a moderate current density. In the simplest case, a ballast resistor stabilizes the electric current. The current is e.g. 10 mA, leading to an electrical power of the order of 10 W. The glass tube as shown in Figure 1 has Brewster windows, and the laser mirrors must form a laser resonator with a small round-trip loss of typically below 1%. Due to the polarization-dependent loss at the Brewster windows, a stable linear polarization is obtained.

Some He–Ne lasers have a tube with internal resonator mirrors, which can not be exchanged. Brewster windows are then not required.

In the gas discharge, helium atoms are excited into metastable states (2³S₁ and 2¹S₀). During collisions, the helium atoms can efficiently transfer energy to neon atoms, which have excited states with quite similar excitation energies (4s₂ and 5s₂). Neon atoms have a number of energy levels below that pump level, so that there are several possible laser transitions. The transition at 632.8 nm (5s₂ → 3p) is the most common, but other transitions allow the operation of such lasers at 1.15 μm, 543.5 nm (green), 594 nm (yellow), 612 nm (orange), or 3.39 μm. The emission wavelength is selected by using resonator mirrors which introduce high enough losses at the wavelengths of all competing transitions.

The 3.39-μm transition involves the same upper laser level manifold as the 632.8-nm transition and exhibits a rather high laser gain, while the 632.8-nm transition. Therefore, 632.8-nm operation is only possible if parasitic lasing on the 3.39-μm line is suppressed by introducing high power losses at that wavelength.

The lower laser level for 632.8-nm operation is still highly excited and is partially depopulated by spontaneous emission. Therefore, one obtains some fluorescence at wavelengths between 0.54 μm and 0.73 μm. This leads to the metastable 3s state; depopulation of that can be made fast enough by using a small diameter laser tube, so that the neon atoms can dissipate energy in collisions with the tube walls. (One also often uses a smaller laser bore tube within a larger glass envelope.) That requirement prohibits simple power scaling via the tube diameter.

The quantum defect is quite high, contributing to a relatively low power conversion efficiency.

The polarization of the laser output is stably linear when Brewster windows are used.

Due to the narrow gain bandwidth (≈1.5 GHz, determined by Doppler broadening), He–Ne lasers typically exhibit few-mode oscillation, or for short laser tubes even stable single-frequency operation, even though mode hopping is possible in some temperature ranges where two longitudinal resonator modes have similar gain.

The lifetime of a helium–neon laser can be far beyond 10,000 hours, since the glow discharge with quite moderate current density is associated with quite moderate operation conditions, e.g. with little erosion of the electrodes. A limiting factor can be leakage of helium. However, the tube may break when being subject to mechanical shock.

Applications of Helium–neon Lasers

Helium–neon lasers, particularly the standard devices emitting at 632.8 nm, are still used for alignment and in interferometers. However, they are more and more replaced with laser diodes, which are much cheaper, more compact and efficient. Remaining advantages of the He–Ne laser can be the smaller emission linewidth (particularly in case of single-mode emission), which is associated with a long coherence length, and the high beam quality.

Some He–Ne lasers are serving in optical frequency standards. For example, there are methane-stabilized 3.39-μm He–Ne lasers, and 633-nm iodine-stabilized versions.

Bibliography