ORANGUTRAN: ORbital ANGUlar momentum TRANsmissometer with zero collection angle error.

通过天然水系统的光经历了吸收和散射，导致了重要的影响，如水的加热，通过光合作用的植物生长，以及为遥感系统产生反射信号。衡量水体光学性质的最常用指标之一是光束衰减系数，它是吸收和散射的总和。这通常是通过记录光束穿过已知长度的水后的强度，并将信号与在空气中或更常见的情况下在超纯水中获得的信号进行比较来测量的。人们通常认为，任何被吸收或散射的光子都不会到达探测器，因此剩余的信号完全是由于直接传输的光子造成的。然而，在现实中，光在水中的散射方式使得标准透射仪意外地收集了大量且非常可变的前向散射光。这意味着它们产生的信号具有很大的误差，这实际上是仪器设计的一个特征，具有不同光学布局的传感器将提供本质上不同的值。长期以来，人们一直认为这是测量的一个不可避免的特征，而大多数用户干脆忽略了这个问题。事实上，NASA目前对这一参数的测量协议明确地将其留给数据的最终用户来解决这个问题。这是一个令人无法忍受的位置，我们最近找到了一个新的解决方案。我们正计划建造一个新的设备来测量光束衰减，该设备利用了最近发展起来的对光子的量子属性的理解，称为轨道角动量，OAM。我们可以控制光的这种量子态，并产生具有定义的OAM态的光束。当这样的光束经历散射事件时，OAM状态会改变一个我们很容易识别的定义的量子量。我们可以利用这种量子态的变化来有效地标记散射光子，并将它们与直接传输的光子区分开来。这意味着我们可以测量在不被吸收或散射的情况下穿过一片水域的光子数量，而不会受到散射收集误差的影响，散射收集误差会给当前的仪器带来问题。届时，我们的设备将比目前可用的设备更准确，并将帮助研究人员和其他最终用户对天然水系统的一种极其重要的光学特性进行显著更好和一致的测量。

Light passing through natural water systems experiences both absorption and scattering leading to important effects such as heating of the water, growth of plants through photosynthesis and generation of reflectance signals for remote sensing systems. One of the most common measures of the optical properties of a water body is the beam attenuation coefficient which is the sum of absorption and scattering. This is usually measured by recording the intensity of a beam of light after it has passed through a known length of water and comparing the signal with that obtained either in air or, more usually, in ultrapure water. It is usually assumed that any photons either absorbed or scattered do not make it to the detector and so the remaining signal is due entirely to directly transmitted photons. However, in reality, light is scattered in water in such a way that standard transmissometers accidentally collect a large and quite variable amount of forward scattered light. This means that the signal they generate has a large error that is actually a feature of the instrument design, and sensors with different optical layouts will provide substantially different values. It has long been thought that this was an inevitable feature of the measurement and most users simply ignore the problem. Indeed, current NASA measurement protocols for this parameter explicitly leave it to the end user of data to work out how to deal with this problem. This is an intolerable position for which we have recently found a new solution.We are planning to build a new device to measure beam attenuation that exploits a recently developed understanding of a quantum property of photons called orbital angular momentum, OAM. We can control this quantum state of light and generate a beam of light with a defined OAM state. When such a beam of light experiences a scattering event, the OAM state changes by a defined, quantum amount that we can easily identify. We can use this change of quantum state to effectively label scattered photons and discriminate them from directly transmitted photons. This means we can measure the number of photons that make it across a volume of water without being absorbed or scattered, without being affected by the scattering collection error that causes problems for current instruments. Our device will then be significantly more accurate than what is currently available and will help researchers and other end-users make significantly better and consistent measurements of what is an extremely important optical property of natural water systems.